This adds support for MSC3289: room version 8. This is room version 7 + MSC3083.

Previously only world-readable rooms were shown. This means that
rooms which are public, knockable, or invite-only with a pending invitation,
are included in a space summary. It also applies the same logic to
the experimental room version from MSC3083 -- if a user has access
to the proper allowed rooms then it is shown in the spaces summary.

This change is made per MSC3173 allowing stripped state of a room to
be shown to any potential room joiner.

Previously m.child.room events in non-space rooms would be
treated as part of the room graph, but this is no longer
supported.

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.