[FIX] Translate description in chatter widget (8.0 lp:1262000)

Fix https://bugs.launchpad.net/openobject-addons/+bug/1262000 :
    When an object which inherits mail.thread with description OBJECT is
    created, a message is posted saying:
        OBJECT created.
    In french, this is translated as:
        OBJECT créé(e).
    The problem is that OBJECT isn't translated.

Add _description to translation exports
Translate _description in mail_thread on create with current user's
context which matches the rest of the create string '%s created'