Parse the `m.relates_to` event content field (which describes relations)
in a single place, this is used during:
* Event persistence.
* Validation of the Client-Server API.
* Fetching bundled aggregations.
* Processing of push rules.
Each of these separately implement the logic and each made slightly
different assumptions about what was valid. Some had minor / potential
bugs.
Fixes a regression from 8b309adb43 (#11660)
and b65acead42 (#11752) where events which
themselves were an edit or an annotation could have bundled aggregations calculated,
which is not allowed.
The `latest_event` field of the bundled aggregations for `m.thread` relations
did not include bundled aggregations itself. This resulted in clients needing to
immediately request the event from the server (and thus making it useless that
the latest event itself was serialized instead of just including an event ID).
Consider the requester's ignored users when calculating the
bundled aggregations.
See #12285 / 4df10d3214
for corresponding changes for the `/relations` endpoint.
The PaginationChunk class attempted to bundle some properties
together, but really just caused callers to jump through hoops and
hid implementation details.
This endpoint was removed from MSC2675 before it was approved.
It is currently unspecified (even in any MSCs) and therefore subject to
removal. It is not implemented by any known clients.
This also changes the bundled aggregation format for `m.annotation`,
which previously included pagination tokens for the `/aggregations`
endpoint, which are no longer useful.
