make_room_admin admin API should check if a user is in the room before sending a PL event on their behalf

[Half-Shot]

Currently it will indiscriminately pull the first user out of the power_level list that has a high enough PL, but it's possible for that user to not be in the room. Given we already pull the membership to check if the requester is in the room, we should check this too.

[y-pankaj]

Hi! I would like to work on this issue.
As far as I get it, I should use room_state.get((EventTypes.Member, admin_user_id)) to check if the user is in the room and raise an error if the user not found below the line:

synapse/synapse/rest/admin/rooms.py

Line 434 in a7882f9

admin_user_id = admin_users[-1]

Also, what error should I raise?

synapse/synapse/rest/admin/rooms.py

Lines 431 to 432 in a7882f9

	if not admin_users:
	raise SynapseError(400, "No local admin user in room")

I guess it should be similar to this with the error statement changed. Also, what should be the error statement raised.

[clokep]

@y-pankaj That seems reasonable. I think we would want to only raise that error if none of the users in the power_level list are able to be used.

[clokep]

I believe this is fixed by #9235.