Allow retrieving the relations of a redacted event. (#12130)

This is allowed per MSC2675, although the original implementation did
not allow for it and would return an empty chunk / not bundle aggregations.

The main thing to improve is that the various caches get cleared properly
when an event is redacted, and that edits must not leak if the original
event is redacted (as that would presumably leak something similar to
the original event content).