Improve InlineComments extension

InlineComments (https://www.mediawiki.org/wiki/Extension:InlineComments) is a relatively new MediaWiki extension that provides a long-missing feature for MediaWiki: the ability to do inline annotation on pages, in the manner of Google Docs, Confluence and other content-management systems. Deliverables Add user mentions, i.e. selecting a user or users to "ping" about the current discussion by typing "@" and then selecting a username from the dropdown (T355942) Make sidebar creation more skin-neutral - currently, the "sidebar" in which comments are displayed is created on somewhat of a skin-by-skin basis; it would be good to have the code handle all skins in a standard way, via CSS and/or JavaScript (T355948) Add correctly-linked username, on comment creation - currently, it is only displayed correctly for comments created before the page was loaded (T355943)