leftover commits

I rebased the branch to origin/next

Refactor plata tags:
Add the possibility to inject a css class name and placeholder text into the form via the template tag.

Fix false positive in form errors:
The form would show errors if you used a formset even if they were none.

Change in order overview:
Context sensitive presentation depending on the tax being included in the price or not.

If there is only one payment method, Select it as default. (it's blank=True so usually no default value)