Hotfixes v0.8.1-r4

Add unique index to room_aliases, remove duplicates on upgrade. Convert some columns back to TEXT from bytea

Have TypingNotificationEventSource.get_new_events_for_user() return a deferred, for consistency and extensibility

Neater implementation of membership change auth checks, ensuring we can't forget to check if the calling user is a member of the room

Postgres does not allow you to continue using a cursor after a DB exception has been raised, so move _simple_insert or_ignore flag out of transaction