Switch to transifex for translations

~~All strings, urls and anchors are now in a single yml files for each language
Translations in html are generated by a jekyll tag plugin
Each html file remain independant of each other~~

Update:
All strings, urls and anchors are now in a single yml files for each language
Translated html pages are generated by a jekyll plugin based on unified templates

This change can allow to handle translations in a more efficient way. Allowing more translators to work without learning technicals behind github, html and such. Making translations updates more obvious and easy to handle. And it allows to have more reviews and teams in charge of each translation.