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 think the main thing was the `MODIFIED_DATE_` field. I could also mock `gmtime` but that seemed silly. This also just seemed a bit more specific, and I think I added more fields during development, so it expanded to nearly the whole thing gradually. I can change if you want though and throw in an `ANY` for the time.

Do you know if Mock is smart about dict comparisons in `assert_called_with`?