Per Google, sites are encouraged to upgrade from `analytics.js` to `gtag.js` for Google Analytics tracking. This commit updates core Discourse to use the new `gtag.js` API Google is asking sites to use. This API has feature parity with `analytics.js` but does not use trackers.
`.js.no-module.es6` files will be transpiled without producing a module.
import/export statements are deliberately not supported, so one would still need to use
`const module = require('moduleName').default.`
Justin DiRose
Joffrey JAFFEUX
Robin Ward
Kyle Zhao