Load JSON-LD content to validate

Why did all this happen?

1. I implemented the JSON-LD validation on submission
2. My test was failing because the setup did not initialize everything as expected - after submission, the session/request was not editable
3. I updated the submission operation to correctly perform the transition from SUBMITTABLE to EDITABLE.
4. Tests started failing because of database ConcurrencyExceptions. This meant, the database changes in the various test setups were actually being performed simultaneously to the (HTTP-)operation-under-test!
5. I re-wrote the way the database changes were performed in the tests. Technical background: https://github.com/quarkusio/quarkus/issues/23845