Change preview view method to use RESTful URL.

cms/djangoapps/contentstore/tests/test_contentstore.py

cms/djangoapps/contentstore/views/component.py

cms/djangoapps/contentstore/views/item.py

cms/djangoapps/contentstore/views/preview.py

cms/djangoapps/contentstore/views/tabs.py

cms/static/coffee/spec/views/module_edit_spec.coffee

cms/static/coffee/src/views/module_edit.coffee

cms/static/coffee/src/views/tabs.coffee

cms/static/coffee/src/views/unit.coffee

cms/templates/edit-tabs.html

cms/templates/unit.html

cms/urls.py