Message Editing: Block attempts to edit other people's edits via `/_matrix/client/v3/rooms/{roomId}/send`

This issue has been migrated from [#5364](https://github.com/matrix-org/synapse/issues/5364).

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I can't quite recall where we left this, so filing an issue to track...

It looks like Synapse currently ignores edits from other senders (those who did not send the original message) when constructing the data to bundled into original event. However edits from others currently still come through on the timeline.

We should block edits from other senders in the timeline as well (or even at send time if possible)