Bug description

SYMPTOM: alice, who is an admin, sends HELLO. subsequently receives 2 copies of all messages. (and still receives messages after sending GOODBYE, but cannot send them)
CAUSE: we relay messages to all subscribers and admins. after HELLO, alice is both, so receives messages twice. (GOODBYE removes alice as admin, not subscriber)
FIX: only allow a user to be an admin or a subscriber, never both

Implementation Notes

create membershipResposiory that uses a new membership model/table
- ensure that member model/table has type field that may be ADMIN or SUBSCRIBER
- in migration:
  - move all existing publication and subscription rows into memberships table (setting type appropriately)
  - dedupe any dual-role admin/subscribers to a single ADMIN membership
if a subscriber is ever made an admin (via ADD command), update their membership type to ADMIN, and leave no trace of the former subscriber
for all HELLO messages, if user is already an admin block command and respond with helpful "i can't do that" message
NOW: admins will not receive dupe messages and GOODBYE will be unambiguous (and only required once)