This could cause a minor data leak if someone defined a non-restricted join rule
with an allow key or used a restricted join rule in an older room version, but this is
unlikely.
Additionally this starts adding unit tests to the spaces summary handler.
This should help ensure that equivalent results are achieved between
homeservers querying for the summary of a space.
This implements modified MSC1772 rules, according to MSC2946.
The different is that the origin_server_ts of the m.room.create event
is not used as a tie-breaker since this might not be known if the
homeserver is not part of the room.
