[IMP] mail: improve notification management for customers

Currently it is impossible in Discuss to know whether an email has been sent
to a customer and whether it failed or bounced. A notified partner has an
entry in the needaction m2m table. In this commit we decorate this table to
add fields about the email notification: is an email sent, did it failed, did
it bounce.

This information is kept only for customers. Internal users does not use this
information. Moreover their notification is deleted once the message is read
in the Chatter. This avoids having a notification table that grows quickly.

Chatter now holds a new icon for email details. It allows to know on a thread
status of emails sent to customers.