resourceloader: Add wgWikiID to mw.config in the startup module
This was previously just added by Echo, but it's generally useful.
We should probably deprecate wgDBname, because wgWikiID is a better wiki
identifier that also works when two wikis have different table prefixes
in the same database, but that'll take some work because a number of
things rely on it right now (including ResourceLoader itself, for its
localStorage keys).
Change-Id: I4d289267991f1f9a8e0710ec6ee5a2131306c510