Add Sentry to client for error handling (#43920)

Co-authored-by: Oliver Eyton-Williams <ojeytonwilliams@gmail.com>

config/read-env.js

@@ -31,6 +31,7 @@ const {
   PAYPAL_CLIENT_ID: paypalClientId,
   PATREON_CLIENT_ID: patreonClientId,
   DEPLOYMENT_ENV: deploymentEnv,
+  SENTRY_CLIENT_DSN: sentryClientDSN,
   SHOW_UPCOMING_CHANGES: showUpcomingChanges,
   SHOW_NEW_CURRICULUM: showNewCurriculum
 } = process.env;
@@ -71,6 +72,10 @@ module.exports = Object.assign(locations, {
     !patreonClientId || patreonClientId === 'id_from_patreon_dashboard'
       ? null
       : patreonClientId,
+  sentryClientDSN:
+    !sentryClientDSN || sentryClientDSN === 'dsn_from_sentry_dashboard'
+      ? null
+      : sentryClientDSN,
   showUpcomingChanges: showUpcomingChanges === 'true',
   showNewCurriculum: showNewCurriculum === 'true'
 });