Topic "all" seems to be illegal - intentional or bug?

It seems to be the case that if one of my handlers is subscribed to topic `all`, then Kapacitor will not launch.

To reproduce:
- Using Kapacitor 1.5
- Using provisioned handlers (i.e. they are packaged into a specific path `KAPACITOR_LOAD_DIR` is supplied)
- Since I'm deploy it fresh every time into a k8s env there is nothing to "reload" - everything is launching from scratch
- One of my handlers is subscribed to the topic `all`

The key error lines are these:
{noformat}
ts=2018-07-19T00:53:35.284Z lvl=error msg="failed to remove missing" service=load err="expected id to be topicID/handlerID"
ts=2018-07-19T00:53:35.284Z lvl=error msg="encountered error" service=run err="open server: failed to reload tasks/templates/handlers: expected id to be topicID/handlerID"
run: open server: failed to reload tasks/templates/handlers: expected id to be topicID/handlerID
{noformat}

I browsed the source code that yields these lines: https://github.com/influxdata/kapacitor/blob/master/services/load/service.go

Throwing darts at the board my hunch is that somewhere under-the-hood the topic "all" is being treated as a special case - perhaps by the restapi.

I couldn't seem to find any documentation about illegal topic names, so if this is intentional then it would be great to add that.

I don't have any particular need for a topic name of "all" so I simply renamed it to "all-alerts" and moved on once I tracked down the issue. However, I decided to open this issue behind in hopes of saving time for future-comers that run into this.

Start with services/load/service.go and reproduce the Kapacitor 1.5 startup using a provisioned handler subscribed to the topic `all`. Trace the `expected id to be topicID/handlerID` error during reload. Done means either the handler loads successfully with that topic or the restriction is documented for future users.

Written by the indexing model from the issue text.