Purpose
=======

The "publish/unpublish" widget is always shown on the frontend as long as the user
is a website_publisher and the object has a "website_published" field.

In some modules (namely website_slides) we need to handle a more refined level of access.

This commit adds a "can_publish" computed field that can be overridden to add some additional
access rules to the object and hide the widget if the user doesn't have the rights to
publish/unpublish.