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.
agreed. I was very tempted to do just that as part of this PR but held off. I will do that at some point though. I'm also waiting as we will potentially soon be reworking some of the ET schema (hopefully).