feat: Allow guest access to backstage.

[feanil]

If the user is not in the openedx org, the login fails because the
user is not in the auth backend database.

The login page now looks like this:

[feanil]

I think there could be better text here but this is the built-in content in the component that backstage provides. If we want to improve it we can do that as a follow-up.

[feanil]

This will need a bit more work because the guest user still has write access.