Issue: Cannot Unsubscribe After Disabling Subscriptions

Let's say a user subscribes to a wiki page via email (or any notification, but email is more serious).

1. User is sent an email update per their subscription.
2. The admin disables wiki subscription functionality. User is currently unaware of this.
3. User wishes to unsubscribe.
4. Rather than using the link in the email, the user wishes to use the subscribe tab or "My Subscriptions" to remove the subscription.
5. User is met with a permission denied message.
6. User believes the site is in violation of CAN-SPAM laws.
7. User reports the violation.

Fixed access to the Subscribe tab. With subscriptions disabled, the user will still have access to the Subscribe tab of any content currently subscribed. The user can also view the "My Subscriptions" page as long as there is still one active subscription. However, in this case the user will now only have the option to unsubscribe, rather than change notification type or frequency.

Still need to make the "My Subscriptions" LINK visible without adding an extra query. There should be an extra user field 'subscribe_left', that gets auto-filled when subscriptions are disabled globally, and that gets reduced every time a subscription is removed while subscriptions are disabled. Once 'subscribe_left' is empty, the user should lose access to "My Subscriptions".