Autofill override fields when "use override" selected

[davidmce]

If the default mapping is correct, but a start date is missing (very common I would expect), you need to override the default mapping.
When you do this, the old mapping should fill the "License URL" box, so the user only has to enter the date.

[davidmce]

This could be done on all override fields. The default mapping could be pre-filled into the override field. If this was done, a clear button would need to be added.

[drtjmb]

Would need to think about this in more detail - currently the empty/non-empty override field is used to indicate the existence or non-existence of an override. So if we start autofilling the field need to make sure that this doesn't cause an override to be assumed when in fact there isn't one.

[drtjmb]

Would need some JS to fetch the current derived value (ajax call or embedded in form) and populate the override field.