Klant should not be an abstract class.

Wanted to make this a mapped superclass at first, but that makes it hard
to easily create a list of all klanten.
Fix couple of twig syntax errors in skeletons.

Used "{{" instead of "~".
Fix Klant model.

- Add missing getters and setters in Klant.
- Create missing Bedrijf mapping for field btw_nummer.
Remove default AppBundle routing.
Create Klant CRUD.
Add missing createCreateForm method from KlantController.

The createCreateForm method was removed from KlantController when
refactoring to three different controllers (Klant, Particulier and
Bedrijf).
Mark correct pane as active when creating a user.

When submitting an invalid form, the correct pane (Bedrijf or
Particulier) will be toggled.
Create a behat feature for Klanten CRUD.