Confirm transcription creation or update through Enter key

Refs https://gitlab.com/arkindex/requests/-/issues/610

Two changes when writing in the transcription textarea in the sidebar:

1. Hitting Shift + Enter should add a new line (as currently possible)
2. Hitting Enter should validate the action (either creation or update)

The modal may not be changed for that behaviour if too complex.

PLease also make a patch for the Arkindex documentation to mention that in the annotation doc.