Make RandomInCat work for cats starting with namespace

So if someone has Category:User:Foo, they can go to
both Special:RandomInCategory/Category:User:Foo, or
just Special:RandomInCategory/User:Foo.

Bug: 53239
Change-Id: I2e67f0463b725ce297eeb5a5de73ccbad826907a
Merge "Make RandomInCat work for cats starting with namespace"