Add "Unfeatured" toolbutton to Article Manager

#### With this PR I'm proposing to add an "Unfeatured" toolbutton in article manager.

It all started when while testing something else I did something probably silly: I set all my articles as "Featured". Then I wished to revert them to "*unfeatured*" and... WTF! we don't have a batch operation for that... have to go to the DB...

Then I decided that probably to have an "Unfeatured" button like we have "Unpublish", would probably be a good idea.

After some research I found how to do that (this PR...), but there is something still not working: I couldn't find a way to have the correct icon displayed in Hathor: the icon is there, in `icon-32-featured.png`, but I really don't understand how to activate it, so...  **some help is needed here!** Thanks!