Fix 4 stale reads in CreateClassification

Closes #507 (closed)

• Creating an element then immediately creating a classification on it could cause a HTTP 400 (element not found)
• Creating an ML class then immediately creating a classification using it could cause a HTTP 400 (ML class not found)
• Creating the same classification twice with a worker_version could cause a HTTP 500 (unique violation)
• Creating the same manual classification twice could cause a HTTP 500 (unique violation, with a different queryset)