Fix 500 error on viewing a group with a custom group schema

display_name was missing from group_form_schema() so if an IGroupForm
plugin with a custom db_to_form_schema() for groups was in use, the
group read template would crash with a 500. Add display_name to the
schema so that the template renders successfully.

ckan/logic/schema.py

@@ -252,6 +252,7 @@ def group_form_schema():
         "capacity": [ignore_missing],
         "__extras": [ignore]
     }
+    schema['display_name'] = [ignore_missing]
     return schema