Include bundled events when requesting relations

[clokep]

In order for MSC3440 to be successful it proposes being explicit about the behavior in MSC2675 that the /relations API should include the bundled relations of each returned event.

The behavior here was changed a few times:

• Aggregations: include the original event in the response from /relations #5595 asked to include the original event (in the original_event field), which was done in Include the original event in /relations #5626. Both of these fields included bundle aggregations (which is a bit odd since you're requesting the relations).
• Don't bundle aggregations when retrieving the original event #5654 stopped bundling aggregations in both the original event and the relations events.

The proposal here is to include them on the relations event. Note that #5654 includes the following:

We also do this when pulling the relation events as well, because edits of edits are not something we'd like to support here.

The code in question lies at:

synapse/synapse/rest/client/relations.py

Lines 232 to 237 in 1f9d0b8

	# Similarly, we don't allow relations to be applied to relations, so we
	# return the original relations without any aggregations on top of them
	# here.
	serialized_events = await self._event_serializer.serialize_events(
	events, now, bundle_aggregations=False
	)

Which asserts that we don't allow relations to relations, but I don't think this is accurate. If this is a problem for edits we might need to solve this a different way.