CSS built-in, RM jQuery, use ResourceLoader

CSS that had to be inserted into MediaWiki:Common.css is now stored at
css/ext.CommunityTwitter.css and loaded with ResourceLoader. The JavaScript
used for the chars-left counter has been rewritten to use reg. DOM instead of jQuery
and has been relocated to js/ext.CommunityTwitter.charsleft.js. The extraneous jQuery
library packaged with the extension has been removed since MediaWiki already provides
the library, and the extension no longer requires it. This JavaScript is also now
loaded through ResourceLoader using OutputPage::addModules, and is now compliant with
the coding conventions specified at https://www.mediawiki.org/wiki/Manual:Coding_conventions/JavaScript

Change-Id: I90da864bde34a2829f08a9fed42fe3a7c92d0823