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.
Jess and I agreed (over IRC) to have the current value as the default and allow them to optionally provide a `source_url` parameter that will override. I'll add that soon.