Redesign and mobile use improvements

Change the design for the Feed Library to make a more clear view of all
available feeds.

Change the base colour and the main navigation with gradients (solid
colours for fallback to come soon).

Fix images for mobile use where some style declarations weren’t
supported.

Fix several Firefox specific positioning issues.

Add a new responsive view for screens greater than 1240px: The main
navigation will be fixed at the screen’s side and the header background
(if available, currently News-only) will be displayed even without
scroll.
Fade the main article on every page view

To simulate more smooth transitions the main article blocks now fade in
after page load.