[e.cash] Add google analytics script

Summary:
Adding the google analytics scripts as outlined here https://nextjs.org/docs/messages/next-script-for-ga
And adding a .env file to store the GA ID as outlined here https://nextjs.org/docs/pages/building-your-application/configuring/environment-variables

Test Plan: preview the site and check the scripts are loading properly

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14290