Add a configuration key to disable deleting branches from the UI
    
    In some sensible pagure deployment, such as a the one running on the top
    of dist-git in Fedora, it is important that the history of git repos are
    not modified and that information isn't lost, this means it is not allowed
    to delete a branch that was created.
    
    This commit introduces a configuration key allowing to disable deleting
    a branch from pagure's UI.
    
    Fixes #2458
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>