final step of the big website overhaul

This is a big overhaul of the build process to make it behave more like a normal Jekyll project, and have it easy to build locally.

• moves all of the translation logic into po4a and jekyll-polyglot rather than the local bash script
• removes _userconfig.yml, just edit _config.yml directly as needed
• cleans up the static assets/ handling by polyglot so that it doesn't fetch them from language subfolders. This should make improved asset caching a lot easier, e.g. by adding an ETag to the href="". Using something like https://github.com/envygeeks/jekyll-assets should be easy now
• Some document improvements, tweaks and bug fixes.

@webDev4 @Bubu @redplanet @uniqx