Add mediawiki.org to default $wgNoFollowDomainExceptions

It doesn't make sense to apply nofollow to links included by default,
for instance on Special:Version. It kind of defeats the purpose of those
links and makes MediaWiki and all MediaWiki wikis less connected and more
sparse, harder to find.

Make some assumptions of SideBarTest explicit so that it tests what is should.

Change-Id: I5e7ad67c0037b099236cf7e1b73ae669ebcec0f1
Merge "Add mediawiki.org to default $wgNoFollowDomainExceptions"