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.
Add logic to map the appservice token to the autogenned appservice user ID.
Add unit tests for all forms of get_user_by_req (user/appservice,
valid/bad/missing tokens)
