Bug 1011604 - [Messages] Update to use <gaia-header>

Please note that we will create header element dynamically for action menu in JS(https://github.com/mozilla-b2g/gaia/blob/master/apps/sms/js/action_menu.js#L92) I'm ok to address this in follow up bug, but you might need to resume this styling first.(Or you could try to fix to use gaia header in JS right in this patch, but the schedule seems tight anyway)