[FIX] Translate description in chatter widget (7.0 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'`