[ADD] mail_message: added model_id field to provide search by applicatio...

...n.

[IMP/FIX]mail: mail.js: hide 'Mark all read' button, when thread is having all the messages makred as read.

 - Task: https://www.odoo.com/web#id=6824&view_type=form&model=project.task&action=333&active_id=131
 - Pad: https://pad.odoo.com/p/r.JmRhE3JrBVAPrpye
Couldn't you do a single search on all message models ?