Extend certificate entity page
    
    Add field for choosing CA when issuing new certificate. Add new item to action menu
    on cert details page which allows user to download the certificate as file.
    
    Part of: https://fedorahosted.org/freeipa/ticket/5939
    
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>