React apparently now checks the properties which are set on DOM elements, and
grumbles noisily about unexpected ones. Update BaseAvatar and RoomAvatar so
that they don't set unrelated properties on the DOM elements.
I've been trying to get some tests working under PhantomJS, which appears not
to support String.codePointAt (which is, to be fair, an ES6 addition). For our
limited usecase, it's easier to implement the functionality from first
principles than to try to polyfill support.
