[IMP] Mass mailings send in cron

1. The merge of the "email_template" module into the "mail" module.
2. The send action of the mass mailing has been moved from the frontend to a cron, because it was too slow to send over 10,000 mails (the user's browser was blocked for 15 - 20 minutes). Mass mailings have now their own process in the kanban view.
3. Mails sent from the mail form are sent immediatly instead of from the mail queue (for instance, when you go to sales > customers > list view > select 2 -3 customers > More > Partner Mass Mailing).
4. Users have now the choice from which mailing list they want to unsubscribe when they click on the unsubscribe link at the bottom of the mail.
5. Mass mailings inherit from their campaign UTMs and mass mailing campaigns are linked to an UTM campaign.
6. Many little improvements