Use a Jekyll filter to localize links

Replaces "/{{ page.lang }}/{{ url }}" with "{{ url | localize }}"
to generate an absolute URL for a page specified relative to the
language specific subsite. Introduces some flexibility since
a not specified lang variable defaults to 'en', and absolute links
like '/en/security' or 'http://site' are not modified.
Experiment with link from /de to /en site
Add missing lang variable