ENHANCEMENT: automatic link child class (has_one parent) in addNewForm

Presets and locks-in the parent HasOne relationship value for a child record (e.g. if we add a city to a country then the user can see that the country is preset and that the country can not be changed (as on save it will be overridden to the parent country anyway).

For a full description and discussion, see https://github.com/silverstripe/silverstripe-framework/pull/2637 and https://github.com/silverstripe/silverstripe-framework/pull/2651