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.
Smart how? Normal dict comparison will work here. Unless you're referring to `ANY`, which doesn't require special handling and can be used in any equality test. So yes?