Remove unused about page
Do code highlighting using highlight.js
Treat rfc822 headers in Markdown posts as case-insensitive
Merge the Post and Page classes

Functionality from Post is moved to Page. The difference is mainly the
presence of tags in posts -- and there is nothing that suggests that having
them in Page is a bad idea.
Load content in kinds, without using the kind as class name

The content kind (as gotten from site.json) was being used as class name.
Now that Page is the only content class, that does not make sense anymore,
so site.content[] has not the content kind as key, and all items are Page
instances.
Add supports for sidebars
Small cosmetic fixes
Import CSS styles for syntax highlighting
Improved site theming and layout
Final take at the initial content sorting

- Interesting posts imported from the Igalia blog are in the year-named
  directories.
- The old/ subdirectory has the content from the Wordpress blog *which is
  interesting* (not all), plus some of the articles from the Igalia blog
  which did not make it to the main section.