[8.0][mass_mailing][FIX]

* Rename `mailing_type` into `mailing model`
* Allow to reply into document that inherit from `mail.thread`
* Add missing fields when creating a new `mail mass_mailing` from `mail.compose.message`
* Repair sql request
Not sure about your change. If the reply_to_mode is using a specified reply-to address, we do not want to display the warning. About removing res.partner from the warned models, well technically it works, but w want to avoid people using the partner page for threading purpose. It should be more an internal discussion thread on the partner.