Do not allow to delete a read-only project
    
    In the case where a project is deleted while gitolite is refreshing its
    Acls, gitolite will re-create the git repo, thus preventing the project
    to be re-created later (or refork in the cases of forks).
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>