Avoid dependencies on content packs from other packages

We currently have circular dependencies between `org.graylog2.contentpacks` and `views`.

In order to produce and consume content packs wrappers need to be mapped to and from our domain entities. For most contexts the mapping can be contained within `org.graylog2.contentpacks`. Mostly this is done in the various `EntityFacade` implementations.

For entities that come from a plugin this doesn't work though, because they can't be referenced from `org.graylog2.contentpacks`. So the mapping code must be included in the plugin instead. The current implementation for event definitions from the enterprise plugin is relying on an interface that's defined in core and implemented by the entity in the enterprise plugin, e.g., https://github.com/Graylog2/graylog-plugin-enterprise/blob/d67881db1f5bda0994d02c44c67ad2cef87c4a2c/enterprise/src/main/java/org/graylog/plugins/events/processor/correlation/CorrelationSource.java#L13

This interface was used also for mapping the entities in `views` to their content pack representations, which is unfortunate, because it introduces a circular dependency, e,g. between `Search` and `SearchEntity` (the content pack representation): https://github.com/Graylog2/graylog2-server/pull/6973/files#r358330395

We decided to leave it like that for the 3.2 release and clean it up afterwards, hence this issue.

## Expected Behavior
The Content Packs code has to know about all entities it has to support, so dependencies on all affected packages are appropriate. Other contexts (like Views, Alerts, etc.) should not depend on content packs to avoid unnecessary coupling in general and circular dependencies in particular.

The mapping for entities coming from plugins must be defined in the respective plugin.

## Current Behavior
See above

## Possible Solution
tbd

## Steps to Reproduce (for bugs)
n/a

## Context
see above

## Your Environment

* Graylog Version: 3.2-SNAPSHOT
* Elasticsearch Version:
* MongoDB Version:
* Operating System:
* Browser version: