Remove globals from special page execution

Finally, all of the globals in the CommunityTwitter special page are
gone, replaced with variables retrieved from ContextSource. With that
done, we should be able to move on to improving the extension's
functionality instead of improving what's already there.

Change-Id: Id7d2984e66bdf998bdd11249abae5824c8a23591
Merge "Remove globals from special page execution"