Allow specifying a random page only in main namespace

Make Special:Random/ get you a page in NS_MAIN. Previously it
got a page in the content namespaces. Special:Random (no /) still
returns pages from all content namespaces.

This also affects Special:RandomRootPage from the RandomRootPage
extension.

Bug: 46420
Change-Id: I9ce66b534b86eaa09236dd56dab7ac18d1ef37ec