py3: fix display of default user permissions

Avoid http://localhost:5000/vcs_test_hg/settings/permissions with explicit user
permissions failing:

  TypeError: '<' not supported between instances of 'str' and 'bool'

kallithea/templates/admin/repos/repo_edit_permissions.html

@@ -12,7 +12,7 b''
                     <td></td>
                 </tr>
                 ## USERS
-                %for r2p in sorted(c.repo_info.repo_to_perm, key=lambda x: x.user.username != 'default' and x.user.username):
+                %for r2p in sorted(c.repo_info.repo_to_perm, key=lambda x: '' if x.user.username == 'default' else x.user.username):
                     %if r2p.user.username =='default' and c.repo_info.private:
                         <tr>
                             <td colspan="4">