Sliding sync: return room tags in account data extension

[erikjohnston]

Room tags in Synapse are in a different table than the standard account data tables. We should add them to the room account data section.

This breaks the favourites tab in Element X (EX).

c.f. the handling in sync v2:

synapse/synapse/handlers/sync.py

Lines 2325 to 2337 in a7fcac5

	ignored_users = await self.store.ignored_users(user_id)
	if since_token:
	room_changes = await self._get_room_changes_for_incremental_sync(
	sync_result_builder, ignored_users
	)
	tags_by_room = await self.store.get_updated_tags(
	user_id, since_token.account_data_key
	)
	else:
	room_changes = await self._get_room_changes_for_initial_sync(
	sync_result_builder, ignored_users
	)
	tags_by_room = await self.store.get_tags_for_user(user_id)