Use relative links and asset paths.

This is needed for github (project?) pages.
Add a link to anaconda-patches list information page.
Use a shorter summary length for blog posts.
Support github project pages' prefix of the project name.

Project pages have a prefix but it isn't treated as a separate namespace.
The URL /about in development becomes /blivet/about in production.
Remove a bunch of unwanted stuff from the default layout.
Switch back to absolute paths for links and assets.

It seemed like the http_prefix ("/blivet/") was being appended to relative
paths, which makes little sense to me.
Convert some html to use rails helpers and thus pick up http_prefix.
Add domain to data/site.yml.
Update introductory blog post.