[REF] mail: mail.thread fields cleaning :

- removed message_summary, a field that computes html containing the number
of unread messages. This field was used only in kanban views. Probably that
in earlier versions of the kanban view this field had a use; however now
we can directly use a counter and use it in the kanban templates.
- message_unread_counter is added, a counter of unread messages computed
in the same time as message_unread. It is now used in kanban views.

Views of several addons have been updated.