Use media queries instead of watching window width

Allow CSS media queries to compute the style of the sidebar.
Make the CSS for the sidebar mobile first.
Rename ```active``` and related names to ```expandable``` to make the intended use clear.

Calling ```$scope.$apply()``` every time ```window.innerWidth``` changes will incur unnecessary recalculations.
As an app gets more complex this could become a performance issue.

Note: The media queries should probably be organized and located in a dedicated section, but they are currently added inline to conform to the existing code style.