Large standardization, primarily for i18n.

Message names have been changed and standardized to facilitate TranslateWiki
translating of the extension.  The special page has accordingly changed to
Special:CommunityTwitter from Special:Twitter, and the class Twitter renamed to
CommunityTwitter and the files likewise renamed.  In addition, the default
installation path has also changed from extensions/Twitter to
extensions/CommunityTwitter.  Lastly, qqq message descriptions were added for
all messages.

Still todo: Standardize $ct globals into $wg globals.

Change-Id: Ice9de29ceae3cbe34f59db3b6b20f032909aeeb3