[IMP] mail, website_mail_group: discussion group improvements

- added possibility in mail to have a model adding custom headers in emails sent for notifications for new messages
- mail.group now add list-id and precedence: list in the headers to inform mailing systems that those mails are to be considered as mailing lists
- website_mail_group adds some further data in the headers (subscribe, unsubscribe, archives)
- groups page now display the number of message in the last month
- notification emails are now queued after 50 recipients