Remove Transcription.zone and attach all transcriptions to a direct element

Requires !870 (merged), closes #310 (closed), closes #50 (closed). Provides the missing field that blocks frontend#355 (closed).

Migrating

Run the migrations as usual. Once completed, you will need to run arkindex reindex --elements as most transcriptions will be reassigned to new elements and the reindexation and search logic have changed. --drop is not needed.