Check if Dashboard-Connected Stream Exists When Installing Content Packs

## Expected Behavior
Say you have a Dashboard in a Content Pack that has been scoped to use only certain Streams. When you install that Content Pack, Graylog will detect the existence of the Stream(s) and the installed Dashboard will use them.

## Current Behavior
When you install a Content Pack that includes a Stream-scoped Dashboard, the Content Pack will create a new Stream with a new ID regardless if one exists already. If one was already present on the system, now you have 2 identically-named Streams but with different ID's.

## Possible Solution
Perform some form of Stream name/ID matching when installing Content Packs that include Streams, and if there is a match, do not create a new Stream and configure any objects connected to that Stream in the Content Pack to use the pre-existing Stream.

## Context
See: https://github.com/Graylog2/graylog-project-illuminate/issues/1227 for context & motivation.

This particularly affects customers migrating to Illuminate and Graylog Security. When the new Security Dashboards are installed, they are configured to search across all Streams because of this potential for Stream name duplication because of this issue.