Add the ability to "mute" a feed

[passiomatic]

It would be cool to have a way to "mute" a feed to avoid its entries to appear in the unread list. Feeds like Dribbble popular are nice to have but they clutter the main "unread" view (too many posted entries). The idea is to still fetch the feed but show its entries only in the single feed view.

One can mute a feed right when it is added to Coldsweat (in the Subscribe modal dialog) or in the feed "About" modal dialog.

[passiomatic]

Mostly done in a7656cf. Need to refine the UI and do proper testing via the Fever API.