Use $wmgUseVForm to toggle VForm for both login & account creation

The existence of two configuration variables that each separately enables the
VForm interface for login or account creation reflects the history of their
development rather than any current need for granularity of control. Since
we're enabling both interfaces on 30 wikis, rather than have two 31-member
arrays (one for each of wgUseVFormCreateAccount and wgUseVFormUserLogin), we
can use a wmg-var that enables both.

Also enables VForm on arwiki, astwiki, cywiki, dawiki, dewiki, enwiki,
enwikiquote, enwikibooks, enwikisource, enwikispecies, enwikivoyage,
enwiktionary, frwiki, frrwiki, glwiki, hewiki, itwiki, kowiki, mkwiki, nlwiki,
plwiki, roa-tarawiki, slwiki, svwiki, viwiki, zhwiki, commons , meta,
mediawiki, and wikidata.

Bug: 46333

Change-Id: If7d33689ce33f107c9d38c65472221fc984f8a81