#77 Simplify transcription model.

Set parent_id as optional and sentence_id required, so that we can both keep
track of what transcription comes from what transcription, if any, and get
all the associated transcriptions of a given sentence in a single query.
 #77 Enforce script unicity.

Allowing multiple scripts can be desirable but comes with a lot of new problems
I don’t want to deal with for the moment:
• If different scripts imply different meanings, from which one do we translate?
• How do we sort them?
• How do we display them? All of them or only the first? Which one is the first?
• Possibly other things I’m not thinking about yet.
 #77 Created and modified is automatically set.
 #77 Draft function to detect target scripts.