v2_alpha /register fixes for Application Services

Namely this means using registration_handler.appservice_register.

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.

Generate local thumbnails on a thread

Pick larger than desired thumbnail for 'crop'

Improve naming

Implement read receipts.

Implement presets at room creation

Change power level semantics.

Add bulk insert events API

Add ability to restrict room history.