Move public room call invite mitigations to `filter_events_for_client`

[anoadragon453]

This bit of code filters out call invite events in public rooms to prevent someone from being able to ring hundreds of people by sending a call event:

synapse/synapse/handlers/sync.py

Lines 934 to 941 in a023538

	loaded_recents = []
	for event in filtered_recents:
	if event.type == EventTypes.CallInvite:
	room_info = await self.store.get_room_with_stats(event.room_id)
	assert room_info is not None
	if room_info.join_rules == JoinRules.PUBLIC:
	continue
	loaded_recents.append(event)

@MadLittleMods asks whether it can be moved to filter_events_for_client rather than its current spot in _load_filtered_recents, so that the new native sliding sync implementation can utilise it (#16908 (comment)). Another alternative is to soft_fail the call invites.