Require zones for CreateElementTranscriptions

Closes #439 (closed)

This prevents 500 errors when trying to create transcriptions on an element without a zone. Note that while CreateTranscription already does not require zones, CreateElementTranscriptions is entirely based on zones and would need extra work to allow creating many transcriptions at once on an element without a zone; the bulk endpoint would need to validate that none of the sent transcriptions have a zone, and would only be able to create transcriptions on the element without creating sub-elements.