Update post layout, tweak icon css, add comments section, new fonts

- Drop the banner config value since the banner is going away
- Rename css class .contact to .social
- Remove post titles from Older/Newer buttons
- Change header/body fonts
- Add dummy comments section for planning Disqus integration
Update pygments theme emacs -> native

Scheme fits better with the upcoming style overhaul for posts
Update color palette, minor layout tweaks

Simplify the palette and add a background for the main post body,
use page background for code background.

Main page: Move post date to the left for better mobile rendering