Distinguish 'client' from 'federation' events (#3658)

Fixes #3305 
Fixes #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.)

data/api/application-service/transactions.yaml

@@ -65,8 +65,7 @@ paths:
                 description: |-
                   A list of events, formatted as per the Client-Server API.
                 items:
-                  type: object
-                  title: Event
+                  $ref: "../client-server/definitions/client_event.yaml"
             required: ["events"]
       responses:
         200: