[REF] mail, crm, hr_recruitment, project, project_issue: simplified repl...

...y-to of notification emails (now displaying 'YourCompany -docname- <email>' instead of 'Followers of ...'; added same_thread directly on the mail.message model to ease the reply_to and message-id computation; factorized a bit the reply-to generation for a batch of records; udpated tests accordingly.