Appservices cannot create rooms with a large invite list

[Half-Shot]

I thought this was fixed in #9663 but it's still a bug as of v1.33.0. Trying to create a room while inviting 7 users caused the homeserver to respond with { errcode: 'M_UNKNOWN', error: 'Cannot invite so many users at once' }

[babolivier]

The issue seems to come from this code within the create_room function:

synapse/synapse/handlers/room.py

Lines 679 to 680 in 4b965c8

	if len(invite_list) + len(invite_3pid_list) > self._invite_burst_count:
	raise SynapseError(400, "Cannot invite so many users at once")

It looks like this is because this code isn't using the RateLimiter class, probably due to the fact that we don't want to check if the user can invite just one user but potentially many of them. So either we should add this kind of capability to the RateLimiter and use it there, or tweak this code path to look up overrides from the database and appservice configs, which sounds like more faff to me.