In a new room version, the "notifications" key of power level events are
subject to restricted auth rules.

... and use it in places where it's trivial to do so.

This will make it easier to pass room versions into the FrozenEvent
constructors.

These are easier to work with than the strings and we normally have one around.

This fixes `FederationHander._persist_auth_tree` which was passing a
RoomVersion object into event_auth.check instead of a string.

Collect all the things that make room-versions different to one another into
one place, so that it's easier to define new room versions.

Make it so that, before there is a power-levels event in the room, you need a
power level of at least 50 to send state.

Partially addresses https://github.com/matrix-org/matrix-doc/issues/1192