Add wysiwyg editor integration for @mentions

[luxaritas]

If you type @ (or presumably hit an @ button in the toolbar or such), it should bring up a dropdown like on the feed page. Once a user is selected, it should render the appropriate hyperlink to their profile. At the same time, we'd replace the (somewhat broken/inconsistent) implementation from our backend with only picking up on the mention to notify the user, but not render the link.