Add "change area"

[lvps]

Allow admins to change area (preference) of a candidate, with a dropdown and a button.

Add another db column, default NULL, for the previous area, and save the old one there when changing. Multiple changes do not need to keep a log, just the current area (obviously) and the previous one.

This just modifies the data in those 2 database columns, nothing else should be changed (but offer a button to cancel the interview somewhere, this may be used after a first interview to schedule a different one in a different area).

The old area should be added to the JSON export for GDPR.