Creating a classification does not return its ID and validation status

Sentry Issue: ARKINDEX-FRONTEND-D7

Error: Request failed with status code 405
  at call (./js/api.js:86:62)
  at u (None:132:3)
  at a (None:132:3)
  at l/</< (None:132:3)
  at l/< (None:132:3)
...
(36 additional frame(s) were not displayed)

Creating a classification on an element in the frontend, then trying to validate or reject it, causes a HTTP 405 in preprod/prod and 404 in dev because the frontend tries to call /classification/undefined/reject/ or /classification/undefined/validate/. This is due to CreateClassification not properly returning the new classification UUID.

Additionally, the frontend does not display the classification as validated, even though manual classifications are automatically validated when they get created, because the endpoint does not return the validation status either.