[#4031] Pass group_type to plugins so they can choose what to do

For instance when rendering the form for a new group/org there is not
way of telling if it's a group, org or custom type. The existing method
for the plugin interface just accepts context and data_dict, so to
maintain backwards compatibility I'm adding the group_type to
data_dict['type'], which is consistent with what they would get on an
update operation.

ckan/controllers/group.py

@@ -53,6 +53,8 @@ def _db_to_form_schema(self, group_type=None):
         return lookup_group_plugin(group_type).db_to_form_schema()
 
     def _setup_template_variables(self, context, data_dict, group_type=None):
+        if 'type' not in data_dict:
+            data_dict['type'] = group_type
         return lookup_group_plugin(group_type).\
             setup_template_variables(context, data_dict)