[FIX] mail: fixed bounce email recognition + invite email headers + mass...

[FIX] mail: fixed bounce email recognition + invite email headers + mass mailing statistics not lost anymore
- [FIX] bounce regex: too many emails were considered as bounce and therefore
not displayed in the chatter and lost for the communication history. The regex
was not correctly looking for the bounce alias in the email_to.
- [FIX] invite email: replying to the invitation email (invitation as new
follower) now replies to the user sending the invitation.
- [FIX] mass_mailing: added a column to store the id of the original email
in addition to the many2one column. The many2one is set to null when deleting
the original email. As the information is necessary, it is saved on another
field. The many2one is necessary for indexes purpose as the inverse of
a one2many.