v3 rails 4.2

Created by: dmargery

Migrate the code base to rails 4.2.10

This was done by 1/ cherry-picking changes made in master to migrate to rails 4.2, 2/ cherry-picking the changes made to the development environment so as to be able to work on the v3 branch 3/ migrating all the stuff related to environment management, not present in master, to rails 4.2 4/ cherry-picking all the minor/convenience stuff integrated into master since v3 was branched (all required to drop stunnel, to be reached through a proxy, to better report standard problems such a grit timeouts, and to add headers to simplify writing api-proxy