Current room upgrade behavior does not work nicely with invite-only rooms

Since `m.room.member` events are ignored, members of the old room have no way of joining the new room unless the person who performed the room upgrade specifically invites them all.

Other cases that should be considered (thanks to kitsune for bringing them up):

- Users who are invited to a room but don't join before a tombstone event is sent and then do join after.
- Users who scroll to the top of an upgraded room after joining it directly and then see a reference to an old version (invite-only) room they aren't part of.
- Admin upgrades an publically joinable room and then immediately makes it invite-only before any previous room members can join.