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.
Other signup methods have a field that's POSTed to basket with this value, right? Why not do that here to, in case a site besides accounts.firefox.com starts signing people up with FxA ids? (That may be really unlikely but probably not impossible. Also helps defend against domain name changes.)