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.
Could do that. I'll ask Jess about it. This field is really just for her analytics, so if she'd like for the users of the method to control that then this would be a good idea.