[Refs #4] Make panels responsive

Add responsive styles to show panel columns as a single row below 768px
screen width.
[Refs #4] Add layout styles for 1220px - 1336px displays

Extend the base grid to 1200px for displays between 1220px and 1336px
[Fix #4] Add layout styles for 1600px and larger displays

This commit adds a new layout for screens larger than 1580px. It refines
the content and sidebar stretching, by letting only the content portion
stretch on larger screens, keeping sidebar at the same width.

Currently the 1560px grid serves 1920px and larger screens.