Fixes#3305Fixes#3380
The idea here is to better distinguish between a 'raw' event (as we send over the wire), and the
'serialised' format, as sent in responses to the C-S api and in `PUT /_matrix/app/v1/transactions/{txnId}`.
It's made more complicated by the fact that there are _two_ serialisation formats, one used by `/sync`
and `/notifications`, and one by everything else (the difference being whether `room_id` is included).
In an ideal world, we wouldn't repeat `SerialisedEvent` every time it's used, and instead just link to the
first reference, but that's a job for another day.
Another job for another day is to get rid of things like `sync_state_event.yaml` (which is now used
only in one place, so should be inlined.)
