Message editing: support editing pending messages

[bwindels]

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[bwindels]

we'd need to emit edit_event with the fake id and listen for Room.localEchoUpdated and reemit edit_event with real event id ... all this without losing the editor internal state 🥴

Also, we can't send the edit until we have a real event id. Should we disable the save button or just queue (1) edit(s)? I guess the normal send queue would be good for this, but we need to modify the replacement event with the real event id after it has gone into the queue.