Trusted clients and servers

[kasperwelbers]

Implement client/server registration.

In server config, the middlecat url should include an id. This id is obtained after registering at middlecat. User can then add settings on middlecat for access to the server. Such as allowed email addresses / email domains, allowed oauth providers, and allowed clients. This has two functions: (1) protect middlecat from unwanted users, (2) can be used by servers as simple authorization.

Clients can also register. Then need to provide ID and Secret, and if so, they're given separate authorization screen with additional details. This can then be used by clients with a backend (with middlecat-react this can be integrated in the BFF). This has the function of becoming proper oAuth2.0.

This also requires "cat person" rights at middlecat. Should add superuser (via env) who can assign this right.

[kasperwelbers]

Also add scopes, if we're are it anyway