Update Content Pack system to handle Streams by name rather than ID

[waab76]

Because Content Packs refer to Streams by ID rather than name, it's functionally impossible to share a content pack with queries/alerts/dashboards that are scoped to a particular stream.

What?

While Graylog doesn't enforce uniqueness for Stream names, our intuition is that most users most of the time are not purposefully creating multiple Streams with the same name. We think it would be great if the Content Pack creation logic could store Stream names in the Content Pack in addition to or in place of the Stream ID. Then the Content Pack installation logic can look up streams by name at install time and fail gracefully if the stream either doesn't exist or if multiple streams exist with the requested name.

Why?

The lack of this feature causes significant pain for users when they install Content Packs (from the community or Illuminate) where a query, event, or dashboard relies on a field that has different type mappings in different indices but no way to limit the query to the intended index set.