Refined a unit test when subscribing a user to a feed in the Feed model.
User can now subscribe to a feed already in the database, submitting its URL (normally the url of the website generating the feed).

Also DRY views,moving the code that generates the markup for a single feed in the sidebar to its own partial. This partial is rendered from several places (when rendering the page for the first time and when dynamically adding a subscribed feed via AJAX).