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.
You can keep this old code because you always overwrite the `fxa_id`. Since you're now always setting and saving it if it exists, there's no need to add `fxa_id` to the defaults.