feat: convert auth0 domain to issuer url

[ojeytonwilliams]

Auth0Provider expects the full URL, but Auth0's dashboard provides the domain. This converts between the two because it's easy to forget.

Checklist:

• [x] I have read freeCodeCamp's contribution guidelines.
• [x] My pull request has a descriptive title (not a vague title like Update index.md)

I'll need to update the deployment pipeline, so please @ me after reviewing.