Installing Content Pack may create new All messages Stream

## Expected Behavior
The "All messages" default Stream is somehow special, and installing a Content Pack that has it in the entity list should not create a new Stream for it, but use the existing one.

## Current Behavior
Installing a Content Pack that has the "All messages" Stream as dependency creates a new "All messages" Stream, that does not behave in the same way as the original one.

That affects Pipelines that may need to be applied into all messages, and specially Dashboard Widgets created against the Stream. The latter case is specially bad, because we don't let users alter the Stream after the Widget was created.

Example of content pack having this issue: [v2.txt](https://github.com/Graylog2/graylog2-server/files/2808711/v2.txt)

## Steps to Reproduce (for bugs)
1. Upload given Content Pack
2. Install the Content Pack
3. Go to the Streams page
4. See that two "All messages" Streams exist

## Your Environment
* Graylog Version: Graylog 3.0.0-rc.1-SNAPSHOT (790939a)