So a Mail Message can be created from a Post, and linked to another Mail Message if it is a response

fixup! erp5_interface_post: add interaction workflow to set message-id as reference of Internet Message Posts

erp5_interface_post: add interaction workflow to set message-id as reference of Internet Message Posts

In case of Emails, several posts will be created if there are more than 1 recipients, so trying to cancel duplicates Internet Message Posts is impossible (or some customers will never receive the emails)

erp5_crm: display standard preview on Mail Message only if internet_message_post_module isn't installed

To prevent race-condition in a multi-node environement (ie: 2 activity nodes load the afterAllowExport activities for 2 Posts having the same reference)