Rename Special:RandomInCategory to Special:RandomPageInCategory

I stole this extensions special page name for a core feature, so
I guess I should rename it here to prevent conflicts.

See https://translatewiki.net/wiki/Thread:Support/About_Special:RandomInCategory

(Note there is a difference between this extension and the core
feature, namely this extension gives much more random results
in exchange for heavier performance cost. So its not exactly
replaced by core functionality.)

I suppose it could use SpecialPage_initList hook to check if
the core page is loaded, and still load the old name for
old versions of mediawiki. Seems easier just to tell people
to use the branched version of this extension.

Change-Id: I52e9b1da3ca91ca611dc3ca50d0c75f2cd94c4e6