Re-implement CreateTranscriptions

It should be way simpler now to create bulk transcriptions on existing elements:

A worker such as kaldi works on existing elements (text lines) and just want to report back their transcriptions.

The worker could simply send a payload of transcriptions:

[
  {
    "element_id": "UUID1",
    "text": "XXX",
    "confidence": 0.2,
   }
...
]