Non-legacy solution for ContentMapper::copyLocale

[dantleech]

The copy locale method (ContentMapper::copyLocale) currently has no counterpart in the DocumentManager API(s).

This command might even be redundant when the routing auto (#1989) integration is made, as this method is mainly concerned with the ReosurceLocator.

I think it should basically be possible as follows (with the routing auto PR):

$document = // ...
$newLocale = 'de_at';
$manager->persist($document, $newLocale);

[danrot]

Replaced by #3227