[MERGE] mail: add various mailgateway improvements

This merge holds several long awaited improvements for the mail gateway.
It includes

 * store email status of notifications emails sent to customers
 * display this information on Chatter
 * improve bounce email management: track bounced notifications emails, use
   message_bounce fields directly in mail and not in mass_mailing
 * send emails to assigned user on records
 * remove required alias on users while keeping it possible to define alias
   on users and use them
 * handle emails forwarded to aliases as new threads instead of replies
 * do not store needaction for forum and blog posts
 * automatically unsubscribe partners that bounce on channels
 * some light code cleaning