Strip styles down to a mobile friendly layout

There has already been some discussion on the minimum resolution Bartik should support. A general consensus is 320px although this is still up for debate. Some phones go as low as 240px.

If we are going to take a Mobile First approach for this, only loading the essentials for mobile users and progressively adding more styling as the screen width increases, then we need to strip away a lot of the styling contained in Bartik and maybe tweak some to create a mobile friendly layout.

This is part of: #1077094: Make Bartik a responsive design - meta issue