[FIX] mail: correctly filter needaction on unread notifications in message_format

Previously to this commit needaction used by JS was not taking into account
read notifications. It means notably Inbox counter could be higher than
expected as taking into account read messages while Inbox displays only
unread messages. This commit fixes that by checking notification state when
giving needaction_partner_ids to message_format result.

This fix has a small impact on performances as message_format is used in
notification process for chat or push. A better implementation of message
format and its postprocess could probably lessen this fix impact. As we
target stable version we avoid rewriting other part of the code. Optimization
will be done in development version.

Related to task ID 1841243.