Hide the ``+`` button on the index page when it is disabled in the UI
    
    We have logic in the master template and in the actual controller to
    disable the ``+`` button at the top and the endpoint to create new
    project when the pagure instance is configured to not allow project
    creation.
    
    Fixes https://pagure.io/pagure/issue/2769
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>