Move to FxA UIDs 

Switch to FxA UIDs instead of SHA256 hash elements. 
(note: 248 supersedes some concerns from 235. Older users are upgraded to FxA UIDs only after they've logged into the FMD site.)

@crankycoder: r?
You probably want to remove the references to 'email' in the comments.  I know I'm going to forget what you're talking about in about a week. :)