Content pack installation does not rollback on failure

## Expected Behavior
After a failure during content pack installation, the installation should rollback all created entities, so the system is exactly as it was before trying to install the content pack.

## Current Behavior
A failure during content pack installation leaves some entities behind. On this case a dashboard.

Server stack trace

```
2019-01-23 17:35:04,383 ERROR: org.graylog2.shared.rest.exceptionmappers.AnyExceptionClassMapper - Unhandled exception in REST resource [3/1941]
org.graylog2.contentpacks.exceptions.ContentPackException: Failed to install content pack <9136f1bb-5d5f-49cb-bc49-9551f28c143a/1>
at org.graylog2.contentpacks.ContentPackService.installContentPack(ContentPackService.java:158) ~[graylog.jar:?]
at org.graylog2.contentpacks.ContentPackService.installContentPack(ContentPackService.java:99) ~[graylog.jar:?]
at org.graylog2.rest.resources.system.contentpacks.ContentPackResource.installContentPack(ContentPackResource.java:294) ~[graylog.jar:?]
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_191]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_191]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_191]
at java.lang.reflect.Method.invoke(Method.java:498) ~[?:1.8.0_191]
at org.glassfish.jersey.server.model.internal.ResourceMethodInvocationHandlerFactory$1.invoke(ResourceMethodInvocationHandlerFactory.java:81) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher$1.run(AbstractJavaResourceMethodDispatcher.java:144) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.invoke(AbstractJavaResourceMethodDispatcher.java:161) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.internal.JavaResourceMethodDispatcherProvider$TypeOutInvoker.doDispatch(JavaResourceMethodDispatcherProvider.java:205) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.dispatch(AbstractJavaResourceMethodDispatcher.java:99) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:389) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:347) ~[graylog.jar:?]
at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:102) ~[graylog.jar:?]
at org.glassfish.jersey.server.ServerRuntime$2.run(ServerRuntime.java:326) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors$1.call(Errors.java:271) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors$1.call(Errors.java:267) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors.process(Errors.java:315) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors.process(Errors.java:297) [graylog.jar:?]
at org.glassfish.jersey.internal.Errors.process(Errors.java:267) [graylog.jar:?]
at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:317) [graylog.jar:?]
at org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:305) [graylog.jar:?]
at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:1154) [graylog.jar:?]
at org.glassfish.jersey.grizzly2.httpserver.GrizzlyHttpContainer.service(GrizzlyHttpContainer.java:384) [graylog.jar:?]
at org.glassfish.grizzly.http.server.HttpHandler$1.run(HttpHandler.java:224) [graylog.jar:?]
at com.codahale.metrics.InstrumentedExecutorService$InstrumentedRunnable.run(InstrumentedExecutorService.java:181) [graylog.jar:?]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [?:1.8.0_191]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [?:1.8.0_191]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_191]
Caused by: org.graylog2.contentpacks.exceptions.ContentPackException: Couldn't create dashboard
at org.graylog2.contentpacks.facades.DashboardFacade.decode(DashboardFacade.java:163) ~[graylog.jar:?]
at org.graylog2.contentpacks.facades.DashboardFacade.createNativeEntity(DashboardFacade.java:141) ~[graylog.jar:?]
at org.graylog2.contentpacks.ContentPackService.installContentPack(ContentPackService.java:149) ~[graylog.jar:?]
... 29 more
Caused by: org.graylog2.contentpacks.exceptions.ContentPackException: Missing stream for dashboard widget "Security exceptions": EntityDescriptor{id=5c474805e0015c16e50bc32e, type=ModelType{name=stream, version=1}}
at org.graylog2.contentpacks.facades.DashboardFacade.createDashboardWidget(DashboardFacade.java:241) ~[graylog.jar:?]
at org.graylog2.contentpacks.facades.DashboardFacade.createDashboard(DashboardFacade.java:189) ~[graylog.jar:?]
at org.graylog2.contentpacks.facades.DashboardFacade.decode(DashboardFacade.java:154) ~[graylog.jar:?]
at org.graylog2.contentpacks.facades.DashboardFacade.createNativeEntity(DashboardFacade.java:141) ~[graylog.jar:?]
at org.graylog2.contentpacks.ContentPackService.installContentPack(ContentPackService.java:149) ~[graylog.jar:?]
(...)
```

Half-created dashboard left behind

```
{
"_id" : ObjectId("5c4897b8e0015c3b11556db5"),
"creator_user_id" : "hans",
"description" : "test",
"created_at" : ISODate("2019-01-23T16:35:04.380Z"),
"title" : "Test dashboard",
"widgets" : [
{
"creator_user_id" : "hans",
"cache_time" : 10,
"description" : "Map",
"id" : "933a5f44-bca8-489a-8740-c45db3edb236",
"type" : "org.graylog.plugins.map.widget.strategy.MapWidgetStrategy",
"config" : {
"timerange" : {
"type" : "relative",
"range" : 300
},
"field" : "src_ip_geolocation",
"query" : ""
}
},
{
"creator_user_id" : "hans",
"cache_time" : 10,
"description" : "Top programs",
"id" : "373f2c7c-6c1c-4610-8090-04e93e40019b",
"type" : "QUICKVALUES",
"config" : {
"timerange" : {
"type" : "relative",
"range" : 300
},
"field" : "program",
"query" : "",
"show_data_table" : true,
"limit" : 5,
"show_pie_chart" : true,
"sort_order" : "desc",
"stacked_fields" : "",
"data_table_limit" : 10
}
},
{
"creator_user_id" : "hans",
"cache_time" : 10,
"description" : "Quick values histogram",
"id" : "58f1ce59-777f-42b7-8ec1-1b13cc933839",
"type" : "QUICKVALUES_HISTOGRAM",
"config" : {
"timerange" : {
"type" : "relative",
"range" : 300
},
"field" : "system",
"query" : "",
"limit" : 5,
"sort_order" : "desc",
"stacked_fields" : ""
}
}
]
}
```

Affected content pack. It was installed on a system where the original entities still existed. I had to use `.log` as extension so Github is happy about it.
[download.log](https://github.com/Graylog2/graylog2-server/files/2788323/download.log)

## Steps to Reproduce (for bugs)
1. Install provided content pack
2. Error occurs during install
3. Go to dashboards page and see the duplicated entity
4. Content pack installation shows "No data available"

## Your Environment
* Graylog Version: Graylog 3.0.0-beta.4-SNAPSHOT (87afab1)