Add avatar uploading via profile

Things of note:
 * Duplicating the current_user into user, to make changes show up in
   both the header and the page
 * Trying to keep the old avatar path working for now
 * Lazily just tacking on ".json" to the /avatar path for now (until the
   above legacy codepath is removed)
 * Not using the crop modal, because it'll break animated avatars