Rich chat

Add toggle-able admin highlighting of chat messages (e.g. choose not to be orange).

Add @username detection.  When you type an "@" followed by any number of characters from the beginning of someone's display name, that token is highlighted with a popover with their profile picture (we could add other stuff in the future).  If your own name is @-mentioned, the token is highlighted yellow, and your titlebar flashes for 5 seconds.

(This branch includes the event-history branch as well).