Hook up backup, password, username changing on new settings page

List of Curiosities:
 * It saves twice — once under the current username, and another under
   the new one.  This is to make Ember Data think it's clean.  Stupid.
 * The computed property for detecting if passwords match twitches
   sometimes.
 * The computed property for canSubmit ignores the current value of
   passwordMismatch.  I have no fucking clue, I triple-checked the
   logic.