OfferteArtikel aantal eenheid toevoegen.
OfferteArtikel aantal eenheid GUI.
Fix invalid form child name when creating new OfferteArtikel
Fix rendering of form when creating new offerte artikel.

Form gets rendered row by row. The aantal_eenheid field was added to the
form and was rendered under the submit button.
Fix loading of existing artikel.

Introduced when renaming extraPrijs to extraPrijsVanToepassing.
Fix breadcrumb link to offerte index.
Use chosen for selectboxes.