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.
Also renaming it `get_external_user_data_mock` would make it more obvious what it's mocking.

I mean I'm nitpicking but It'd help me, at least. <3