Show an alert if there's a problem unsubscribing from a feed.
After unsubscribing a user from a feed, if there are no more users subscribed, totally delete the feed from the database.

This in turn launches a delete of all its entries.
Unsubscribe button is hidden and disabled while reading a whole folder (a "Read all subscriptions" link is selected in the sidebar). It's only shown and enabled while reading a single feed.

Ensure that clicking on Refresh and Unsubscribe buttons does nothing if the button is disabled.