plone.app.discussion is an official Plone core package,
thus their translations belong to plone.app.locales.
This commit removes the plone.app.discussion domain and changes it for
plone.
This fixes:
https://github.com/plone/plone.app.discussion/issues/66
- In order this to work with plone.contentrules use commented object as
the main context within custom discussion events and add comment as the
second parameter.