Hide timezone warning sometime errors

[sentry-io]

The code to hide the timezone warning is called on every page, but the warning itself isn't included on every page (e.g. the sign in page). This causes an error on these pages.

The error was introduced when we migrated away from jquery where such calls failed silently when the query selector yielded zero results.

Sentry Issue: DODONA-FRONTEND-2

TypeError: Cannot read property 'classList' of null
  at checkTimeZone (./app/assets/javascripts/utilities.ts:80:32)
  at None (/en/series/2897/overview/:583:10)