feature: allow admin accounts that arent hosted by the given tranquil instance

allowing adding identities/accounts as admins even if they arent hosted on that PDS would be good for people that manage several PDSs, people who host a bigger community PDS but have their personal account on a personal PDS, etc

this would require allowing admin endpoints to be called with service auth. it would probably be good to also decouple the frontend even more so cross PDS admins can log in to it as well