[REF] mail: model update for NewChatter

Main changes :
- mail.notification model is removed. People do not receive notifications
anymore. Instead two ways of following documents exist
  - using a channel; messages will be displayed on the channel itself
  in a near future commit
  - following with its partner; messages will all be considered as
  needaction, using a new m2m table. People should receive less
  needaction messages by following less records by themselves.
There is no more read / unread state anymore. Instead only needaction
messages are considered. Todo (Favorites) messages still exist, and are
stores on a new m2m table instead of using decorated notifciations.
- the main filter for documents is not message_unread anymore, but
message_needaction. A lot of views and filters have been updated
accordingly.
- the vote feature has been removed