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.
V2 Registration forced everyone (including ASes) to create a password for a
user, when ASes should be able to omit passwords. Also unbreak AS registration
in general which checked too early if the given username was claimed by an AS;
it was checked before knowing if the AS was the one doing the registration! Add
unit tests for AS reg, user reg and disabled_registration flag.