In smartphones, animate the showing/hiding of the sidebar.

This way it's much more obvious for the user what happens when he goes to the entries list or to the sidebar.

This is achieved with CSS transitions.
Removed the 300ms delay in hiding the sidebar, in smartphones.

This delay was intended to give the user time enough to see the link (feed or folder) he had clicked on become active (white text on blue background). Since a 300ms animation has been implemented to hide and show the sidebar, this is no longer necessary.
Changed class name. The -xs suffix has been added to better reflect that the class only has effect under the xs bootstrap cutoff.