Add dotenv file loading

lib/gtag.js

@@ -1,5 +1,3 @@
-export const GA_TRACKING_ID = 'UA-139582634-1';
-
 // https://developers.google.com/analytics/devguides/collection/gtagjs/pages
 export const firePageView = url => {
   if (!window.gtag) {
@@ -7,7 +5,7 @@ export const firePageView = url => {
     return;
   }
 
-  window.gtag('config', GA_TRACKING_ID, {
+  window.gtag('config', process.env.GA_SECRET, {
     page_path: url,
   })
 };