This is meant to be an *almost* non-functional change, with the exception that
it fixes what looks a lot like a bug in that it only calls
`auth_handler.add_threepid` and `add_pusher` once instead of three times.
The idea is to move the generation of the `access_token` out of
`registration_handler.register`, because `access_token`s now require a
device_id, and we only want to generate a device_id once registration has been
successful.
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