Switch to check in/out permissions

[rpatterson]

IIUC, plone.app.iterate defines plone.app.iterate.CheckOutContent and plone.app.iterate.CheckInContent permissions but nothing uses them. The @@iterate_control view doesn't check any permission on checkout but zope2.View and manually checks cmf.ModifyPortalContent in Python on checkin. This means it's not possible for a project that requires customizing the check in/out policy, such as one that wants to allow users who can't edit the original content to be able to review and approve checkouts, to do so without overriding the control view and repeating a bunch of conditions.

IMO, the @@iterate_control view should check the currently unused plone.app.iterate.CheckOutContentandplone.app.iterate.CheckInContent` permissions and not use the core CMF permissions.

[djay]

Could help with the issue in #59 which is really about enforcing a review process and disallowing the ability to edit a published object. You would have to adjust the workflow so published state removes the edit permission and adds a checkout permission instead.

[rpatterson]

Will someone review #68?