Expose server name in rc feed

Reviewers of Id819246a9 (Tim, Timo, Chad) have commented that it
would be preferable to identify wikis by their canonical hostname
rather than the value of $wgDBname. Exposing it here so that
the rcstream server can filter subscriptions by it.

Change-Id: I503a0134a8613daddaae9d8df29a1e27fbfd93bc
Merge "Expose server name in rc feed"