Improved install.fr.md page

* Fix latest version
* Translate OPAM usage
Merge pull request #407 from lefessan/install-french

Improved install.fr.md page
Improve French version of mailing_lists.md
Merge Lefessan's contributions to the French Mailing List page
Add translation "buttons" to the front (core) page
Merge branch 'master' into index.fr.md
Fix to use standard translation links