Implement MSC3925: changes to bundling of edits (#14811)

Two parts to this:

 * Bundle the whole of the replacement with any edited events. This is backwards-compatible so I haven't put it behind a flag.
 * Optionally, inhibit server-side replacement of edited events. This has scope to break things, so it is currently disabled by default.

synapse/events/utils.py

@@ -403,6 +403,14 @@ class EventClientSerializer:
     clients.
     """
 
+    def __init__(self, inhibit_replacement_via_edits: bool = False):
+        """
+        Args:
+            inhibit_replacement_via_edits: If this is set to True, then events are
+               never replaced by their edits.
+        """
+        self._inhibit_replacement_via_edits = inhibit_replacement_via_edits
+
     def serialize_event(
         self,
         event: Union[JsonDict, EventBase],
@@ -422,6 +430,8 @@ class EventClientSerializer:
                into the event.
             apply_edits: Whether the content of the event should be modified to reflect
                any replacement in `bundle_aggregations[<event_id>].replace`.
+               See also the `inhibit_replacement_via_edits` constructor arg: if that is
+               set to True, then this argument is ignored.
         Returns:
             The serialized event
         """
@@ -495,7 +505,8 @@ class EventClientSerializer:
                 again for additional events in a recursive manner.
             serialized_event: The serialized event which may be modified.
             apply_edits: Whether the content of the event should be modified to reflect
-               any replacement in `aggregations.replace`.
+               any replacement in `aggregations.replace` (subject to the
+               `inhibit_replacement_via_edits` constructor arg).
         """
 
         # We have already checked that aggregations exist for this event.
@@ -518,15 +529,21 @@ class EventClientSerializer:
         if event_aggregations.replace:
             # If there is an edit, optionally apply it to the event.
             edit = event_aggregations.replace
-            if apply_edits:
+            if apply_edits and not self._inhibit_replacement_via_edits:
                 self._apply_edit(event, serialized_event, edit)
 
             # Include information about it in the relations dict.
-            serialized_aggregations[RelationTypes.REPLACE] = {
-                "event_id": edit.event_id,
-                "origin_server_ts": edit.origin_server_ts,
-                "sender": edit.sender,
-            }
+            #
+            # Matrix spec v1.5 (https://spec.matrix.org/v1.5/client-server-api/#server-side-aggregation-of-mreplace-relationships)
+            # said that we should only include the `event_id`, `origin_server_ts` and
+            # `sender` of the edit; however MSC3925 proposes extending it to the whole
+            # of the edit, which is what we do here.
+            serialized_aggregations[RelationTypes.REPLACE] = self.serialize_event(
+                edit,
+                time_now,
+                config=config,
+                apply_edits=False,
+            )
 
         # Include any threaded replies to this event.
         if event_aggregations.thread: