Allow a user to moderate a transcription

See https://gitlab.com/arkindex/requests/-/issues/382

Not a quick win, but should not be too long/harduous.

When transcribing an element, every pre-existing transcriptions from a worker should have button "Copy From" instead of the edit one for manual transcriptions. This action would:

• immediately create a new manual transcription for the user, using the exact same text
• allow them to edit the manual transcription if needed