for #8358 (Automatically check PO files in all our repositories) a new
jenkins-tools Git repo has been created, and the check_po script has
been copied there. It would be impractical to have this script live in
two different Git repositories, so this branch:
* adds the jenkins-tools repo as a submodule on the main Git repo;
* replace the current wiki/src/contribute/l10n_tricks/check_po.sh
with a symlink to submodules/jenkins-tools/slaves/check_po, so that
the workflow of translators and RMs isn't affected too much;
* updates the translators doc accordingly.
