Add event to monitor changes in the storage of the JWT

Another window could delete the JWT token. Therefore, the storage has to be monitored for changes in the token.

Cookies, localStorage and sessionStorage have events to monitor changes:

• https://developer.mozilla.org/en-US/docs/Web/Events/storage
• https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/cookies/onChanged