No longer persist XModule templates

Instead, we use XModule field default values when creating an empty
XModule. Driven by this use case, we also allow for XModules to be
created in memory without being persisted to the database at all. This
necessitates a change to the Modulestore api, replacing clone_item with
create_draft and save_xmodule.

common/lib/xmodule/xmodule/tests/test_xml_module.py

@@ -141,6 +141,7 @@ class EditableMetadataFieldsTest(unittest.TestCase):
     def get_xml_editable_fields(self, model_data):
         system = get_test_system()
         system.render_template = Mock(return_value="<div>Test Template HTML</div>")
+        model_data['category'] = 'test'
         return XmlDescriptor(runtime=system, model_data=model_data).editable_metadata_fields
 
     def get_descriptor(self, model_data):