Allow per-project checkins emails destination

We would need to customize the mailing list used for Git checkins notifications for customer projects.
For instance, do not send https://github.com/nuxeo/sharp-cpo email notifications to interne-checkins@lists.nuxeo.com but to a dedicated mailing list (it's a customer project, not a Nuxeo one).

=> https://qapriv.nuxeo.org/githooks/send-email hook has to be accordingly adapted; here's a suggestion:

• export properties out of the script (SENDER = "noreply@github.com", RECIPIENT = "ecm-checkins@lists.nuxeo.com", IGNORED_BRANCHES = ["stable"], IGNORED_BRANCHES_SUFFIXES = ["-SNAPSHOT"]) to an INI file
• allow per-project properties