Bug 1051806: Add /fxa-register endpoint.

@Sancus r?

This may conflict a bit, or at least could benefit from, the work
you're doing to move more ET stuff to tasks, but we can always clean
up after all the merging.
I'm not following you... Here are the scenarios I see for this method:

1. User is found  
  Add fxa_id to user and save
2. User is not found.  
  User will be created with fxa_id in one query.

I add it to the defaults so that I don't have to do a 2nd query to update the newly created row if it was created.