[mielecloud] Fix OAuth service handle

[jlaur]

This changes the OAuth service handle from the configured e-mail address to the bridge UID in order to avoid conflicts in the global namespace of handles in the OAuth storage.

Automated migration is not supported by the core classes, therefore the logic is as follows:

• If the new service handle is found, it will be used.
• Otherwise, if an old service handle is found, it will be used, and the user will be advised to re-authorize (through logging).
• On successful re-authorization, any old service handle will be removed from the OAuth storage.

Additionally a clickable link will now be provided when authorization is missing.

Fixes #14783
Fixes #18265