Fix edit links on special pages

Added wgIsArticle to isViewPage checks, otherwise we were attempting
to load VE dynamically on Special:Move/Delete, which resulted in a
broken h1 title and odd behavior off the 'Read' tab.

Also added !wgIsArticle to pageExists. This is a bit of a hack because
we don't have any info on page existence in the JS on these pages
(I think?). But it's better to display 'Edit' for a page that doesn't
exist, than 'Create' for a page that does.

Bug: 49000
Change-Id: Ib47e5524d41e6066b362e0f5645750c769de5193