Return a slug-friendly identifier

def plotly_app_identifier(name=None, slug=None, da=None, postfix=None):
    'Return a slug-friendly identifier'

    da, app = _locate_daapp(name, slug, da)

    slugified_id = app.slugified_id()

    if postfix:
        return "%s-%s" %(slugified_id, postfix)
    return slugified_id