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?
The error from DeleteDevice looks like it's discarded.  On line 1459, we didn't capture the error there either - so I'm not sure if this is an actual problem or not, but it looks like we capture the error without ever doing anything with it.