redirect ac-discovery=1 to the index

This is a temp fix because rkt doesn't follow redirects.

endpoints/web.py

@@ -527,6 +527,9 @@ def redirect_to_repository(namespace, reponame, tag):
   permission = ReadRepositoryPermission(namespace, reponame)
   is_public = model.repository_is_public(namespace, reponame)
 
+  if request.args.get('ac-discovery', 0) == 1:
+    return index('')
+
   if permission.can() or is_public:
     repository_name = '/'.join([namespace, reponame])
     return redirect(url_for('web.repository', path=repository_name, tag=tag))