Support m.room.tombstone states and predecessors

[xetem]

At least as presented in the UI, rooms that have been upgraded and have left an m.room.tombstone event do not seem to forbid the addition of new messages, nor do they redirect to the replacement room. Replacements rooms also do not have any indication that they originated from another room (to view history).

I'll also note that Element/Element X completely replaces any instances of the tombstone'd room with it's replacement (removing duplicates) in the UI, so that Spaces always have the most up-to-date version of a room showing, and that upgraded Spaces are hidden by default.