Fix Issue#236: Can't Unsubscribe From Feed (Feed -> Unsubscribe). 

Issue #236: This feature was as yet unimplemented in Beta 3.

It allows a feed to be temporarily unsubscribed to, which will preserve previously fetched articles but no more. To signify this change, the name of the feed will be greyed out and the circling progress indicator will not turn next to this feed.

The feed can be resubscribed to be selecting the menu item again.

This changeset does not amend the typical behaviour of deleting a feed, which is permanent and removes prior articles from that feed.