server: Discovery endpoints status code mismatch #625

The current implementation of the Discovery API (server/app/interfaces/discovery.py) responds in two scenarios with status codes differing from the specification v3.1.1.

1. POST /lookup/shells/{aasIdentifier}: This endpoint should respond with 201 on success (currently 200)
2. DELETE /lookup/shells/{aasIdentifier}: This endpoint should respond with 404 if no links exist for the specified AAS (currently always 204)

Although these are minor deviations, we should be consistent to the specification and therefore adapt the behavior.