Alerts: Admin user must link account to subscribe to alert

An admin user must explicitly link to an account in order to subscribe to alerts
for that account. If the user is an admin, the user should be allowed to
subscribe to any alert for any account. Note: the user can seemingly subscribe
to it using the bell icon (first column), but when loading the Subscribe Other
Users dialog using the "Bell Plus" icon (far right column), the user is not
shown in the list of available subscribers. The bell icon erroneously shows the
user that he/she is subscribed, when he/she indeed is not.
This should be fixed in the Alert Service.