Replace MSC3266 with MSC2946 for displaying a Room / Space Summary

[callahad]

Pull request #219 used MSC3266: Room summary API to display information about rooms. That MSC has not yet passed FCP and is not enabled by default on homeservers.

However, MSC2946: Spaces Summary has passed FCP and is enabled by default in Synapse.

It should be possible to use MSC2946 instead of MSC3266 to get the same information, but in a way that is formally part of the Matrix spec, and thus more widely supported.

[t3chguy]

@callahad MSC2946 requires an access token, matrix.to functions entirely unauthenticated.

[callahad]

Hah! Thanks. I'll relay that back to the SCT, where the idea for this change originated :)