Refactor DefaultGroupForm's form_to_db_schema_options()

The form_to_db_schema() methods of IGroupForm plugins were not being
called if those plugins inherited from DefaultGroupForm. Refactor it in
the same way as DefaultDatasetForm's form_to_db_schema_options() method
was recently refactored.  Make it call self.form_to_db_schema so that
the form_to_db_schema() methods of IGroupForm extensions get called.
Also make it call new form_to_db_schema_api_create() and
form_to_db_schema_api_update() methods which could potentially be
overridden by extensions also.

ckan/lib/plugins.py

@@ -349,9 +349,6 @@ def history_template(self):
     def group_form(self):
         return 'group/new_group_form.html'
 
-    def form_to_db_schema(self):
-        return logic.schema.group_form_schema()
-
     def form_to_db_schema_options(self, options):
         ''' This allows us to select different schemas for different
         purpose eg via the web interface or via the api or creation vs
@@ -366,11 +363,20 @@ def form_to_db_schema_options(self, options):
 
         if options.get('api'):
             if options.get('type') == 'create':
-                return logic.schema.default_group_schema()
+                return self.form_to_db_schema_api_create()
             else:
-                return logic.schema.default_update_group_schema()
+                return self.form_to_db_schema_api_update()
         else:
-            return logic.schema.group_form_schema()
+            return self.form_to_db_schema()
+
+    def form_to_db_schema_api_create(self):
+        return logic.schema.default_group_schema()
+
+    def form_to_db_schema_api_update(self):
+        return logic.schema.default_update_group_schema()
+
+    def form_to_db_schema(self):
+        return logic.schema.group_form_schema()
 
     def db_to_form_schema(self):
         '''This is an interface to manipulate data from the database