Follow-up fixes to bridging admin power levels MR !2343
(https://gitlab.com/gitterHQ/webapp/-/merge_requests/2343)
1. When someone joins the room, if we add them as an admin of the Matrix room, also add them as an admin of the historical Matrix room
1. Make sure to call `cleanupAdminsInMatrixRoomIdAccordingToGitterRoomId(...)` alongside `addAdminsInMatrixRoomIdAccordingToGitterRoomId(...)` when we `ensureCorrectRoomState(...)`
1. Refactor to use `ensureStateEvent(...)` instead of `ensureStateEventAsMxid(...)` as the bridge user