Move space summaries out of "MSC land"

[S7evinK]

While we already have MSC2946, it is still needed to manually enable space summaries.
We should move it to the fitting spots (e.g. clientapi/federationapi/roomserver), given it is in the spec since version 1.2.
It's also a source of confusion for new users, having to enable the MSC manually.

[swedgwood]

Space summaries are no longer hidden behind an MSC flag :) (#3134)