[#3005] Add back c.group_admins on group read teamplate

It's used by the legacy templates to show a list of the group's admins
in the sidebar. Legacy templates do not support the new group admins
page. This fixes a frontend test that was failing while testing the
legacy templates.

ckan/controllers/group.py

@@ -131,6 +131,10 @@ def read(self, id):
 
         context['return_query'] = True
 
+        # c.group_admins is used by CKAN's legacy (Genshi) templates only,
+        # if we drop support for those then we can delete this line.
+        c.group_admins = self.authorizer.get_admins(c.group)
+
         limit = 20
         try:
             page = int(request.params.get('page', 1))