Make the methods to tag/untag an object work on Issue and PR objects
    
    This allows us to reuse most of the code or at least be consistent in
    our path.
    There are still some differences between issues, pull-requests and
    projects but the code should be able to handle all of these fine now.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>