(bug 40660) Don't mangle   in on-wiki JS.

(bug 40660) Don't mangle   in on-wiki JS.

Fixed by no longer using MessageCache::get() in
ResourceLoaderWikiModule. The only reason I see for using it is to make
default values for .js pages in the MediaWiki: namespace work (by
specifying them in an .i18n.php file), but that's evil and no one was
doing that other than putting in descriptive comments like
/* Site scripts go here */. That, and the previous implementation of
MediaWiki namespace JS that RL replaced did it, so I kept it at the
time, assuming it must have been needed for some reason.

Change-Id: Ica7f35fefcc200f0b3daf1d06d98a0b8ea878f3a