`m.room.redaction` has `redacts` at the event level (SPEC-164)

[matrixbot]

Submitted by @​kegan:matrix.org
m.room.redaction has the redacts key at the top level of the event. Is this really necessary? Given nearly every other event doesn't do this, it would be great to move it to content.redacts instead.

(Imported from https://matrix.org/jira/browse/SPEC-164)

[richvdh]

I think I want this to stay open until we've actually delivered a room version that fixes it