PLAT-2028 - Create mailing list retirement API endpoint

- Removes "email-optin" UserOrgTags for the user
- Creates and uses a new "UserRetireMailingsSignal" signal
- Creates and uses a new "CanRetireUser" permission
- Creates and uses a new setting "RETIREMENT_SERVICE_WORKER_USERNAME"
- Creates a signal handler to globally opt-out the user from Sailthru