[MERGE] [FIX] mail_gateway: better algorithm to find the partner of an incoming email when having multiple matching partners for a given email address.
The mailgateway tries to find a partner that is also an user with the email_from.
If none is found, it takes the first partner with matching email.
In message_post, it tries to find the author based on the document's followers.
Indeed it is very likely that an answer comes from a follower of a document.
The whole process is not done inside the mailgateway because document
and followers related stuff belong to message_post, not to the mail gateway.
Tests have been added in mail.