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

Let 2015 be a year without modelines!
cf #4174

When sending a message with the "Compose new message" button on the right of the user menu, in the top bar, if you tried to save the message as template, you had a traceback because model field of email.template is mandatory, but was set to True because there is no model in such a case.

As there is no any relevant model in such a place, and that the field is mandatory, mail.message is pretty convenient as the default value.

Fields of html type are already escaped in database and the method
`append_content_to_html` defaults the argument `plaintext` to True,
then calls `plaintext2html` if this argument is True, resulting in
the html signature being escaped twice.

This was added as a security check, but access to this field is need by the wizard itself.
As non employee users can't read/create email templates, this will not be problematic