feat(google-analytics): migrate to gtag.js with dual tagging

[caugner]

Summary

(MP-951)

Problem

We have been using analytics.js, which is being deprecated (see here), and we want to smoothly transition from UA to GA4 using dual tagging (see here).

Solution

Migrate to gtag.js, and support multiple measurement ids.

---

How did you test this change?

Kicked off a stage deployment.

Ran BUILD_GOOGLE_ANALYTICS_MEASUREMENT_ID=foo,bar yarn tool google-analytics-code and manually checked the result in client/build/static/js/gtag.js:

// ...
if (Mozilla && !Mozilla.dntEnabled()) {
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());
  gtag('config', 'foo', { 'anonymize_ip': true });
  gtag('config', 'bar', { 'anonymize_ip': true });

  var gaScript = document.createElement('script');
  gaScript.async = true;
  gaScript.src = 'https://www.googletagmanager.com/gtag/js?id=foo';
  document.head.appendChild(gaScript);
}

[github-actions]

This pull request has merge conflicts that must be resolved before it can be merged.

[caugner]

FWIW Dual tagging looks fine on stage: https://developer.allizom.org/static/js/gtag.js

[argl]

Is the ID actually being checked?

[argl]

Looks fine, just one tiny comment/question