Remove duplicate implementations of "who can invite" logic

[MadLittleMods]

Spawning from #19321 (comment),

We have two functions which try to find the users who can invite:

1. get_user_which_could_invite()
2. get_users_which_can_issue_invite(...)

Disparate implementations cause problems

Before #19321, for v12 rooms:

• get_users_which_can_issue_invite(...) would indicate a local user was available.
• But get_user_which_could_invite(...) would find no eligible user due to flawed logic.

This mismatch caused #19120, resulting in a user-facing error. If we instead were working from the same source of truth, a flawed implementation would only result in falling back to the slower but successful remote join instead of the most correct local join.

synapse/synapse/handlers/room_member.py

Lines 1318 to 1341 in 6e80f2c

	allowed_servers = get_servers_from_users(
	get_users_which_can_issue_invite(current_state)
	)
	# If the local server is not one of allowed servers, then a remote
	# join must be done. Return the list of prospective servers based on
	# which can issue invites.
	if self.hs.hostname not in allowed_servers:
	return True, list(allowed_servers)
	# Ensure the member should be allowed access via membership in a room.
	await self.event_auth_handler.check_restricted_join_rules(
	state_before_join, room_version, user_id, previous_membership
	)
	# If this is going to be a local join, additional information must
	# be included in the event content in order to efficiently validate
	# the event.
	content[
	EventContentFields.AUTHORISING_USER
	] = await self.event_auth_handler.get_user_which_could_invite(
	room_id,
	state_before_join,
	)

Refactor

We should refactor get_user_which_could_invite(...) to use get_users_which_can_issue_invite(...) under the hood.

[vbhavh]

.take

[Madhan-1000]

Well I' d like to work on this can you assign it to me ?
I am looking to gain some experience in python .