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.
With this many decorators I actually prefer setting up the mocks in a `setUp` method and assigning them to `self.mock_blah` so that you don't have to manage/alter the arguments for all of your test methods.