Bug 1015297 - [Settings] Update to use gaia-header

One of our criteria is trying to remove `settings.js`, so based on my previous comment in `screen_lock_passcode.js`, let's remove this function and keep them in `SettingsUtils`.

After doing so, you have to call `SettingsUtils.runHeaderFontFit(gaiaHeader)` in `panels/root/panel.js` here : https://github.com/wilsonpage/gaia/blob/master/apps/settings/js/panels/root/panel.js#L28