Introduce wgRelevantPageIsProbablyEditable

Introduce wgRelevantPageIsProbablyEditable

This is equivalent to the existing variable wgIsProbablyEditable and in
keeping with wgRelevantPageName/wgRelevantArticleId; this is done as an
alternative to making a breaking change to the semantics of the former,
as that might cause issues for users who expect wgIsProbablyEditable to
return true on [[Foo]], but false on [[Special:Move/Foo]] and similar.

Change-Id: I6c6ca1cfd93e7be917952980f1e1d57aec3a1292