Fixes content width in Firefox

This adds more responsive styles and fixes and issue where the main
content column was inheriting the window width instead of the column
width in Firefox.
Change featured image size to span the full width

This allows the featured image on galleries and image posts to span the
full width of the column. Previously the images were constrained to
550px wide while the column could be 1200px wide.
Add additional highlighting to hover and active menu states