Events which are annotations/edits may incorrectly get bundled aggregations applied

[clokep]

There's a comment that events which are annotations (ie, reactions) or replacements (ie, edits) should not have bundled aggregations included with them. This behavior was regressed in #11660 as it causes the check for the event type to be skipped when calculating edits:

synapse/synapse/storage/databases/main/relations.py

Lines 661 to 667 in 8b309ad

	# Do not bundle aggregations for an event which represents an edit or an
	# annotation. It does not make sense for them to have related events.
	relates_to = event.content.get("m.relates_to")
	if isinstance(relates_to, (dict, frozendict)):
	relation_type = relates_to.get("rel_type")
	if relation_type in (RelationTypes.ANNOTATION, RelationTypes.REPLACE):
	return None

Subsequently this was also made buggier for threads in #11752 for similar reasons.

(Note that the behavior about whether these events can have relations to them or not, and whether those should be bundled, isn't clearly defined.)

Noticed in #12273.

[clokep]

I have a fix on this which depends on #12273.