Rethink multiwiki support

Merge the functionality of the multiwiki module into the mediawiki
module. All wikis now share a single Apache vhost, installed extensions
and basic configuration.

Roles which were previously using ::multiwiki::wiki to create new wikis
should now use ::mediawiki::wiki. The former ::multiwiki::settings and
::multiwiki::extension resources have been merged with their ::mediawiki
equivalents. Both ::mediawiki::settings and ::mediawiki::extension now
take an optional 'wiki' parameter to specify a target wiki. If omitted
the resource applies to all wikis. The 'wiki:something' title syntax
works to set the target wiki for both resources as well.

Bug: 68605
Bug: 68168
Change-Id: I26b3705726c52cd19f8a727cc2d6f3e956336b2d