Fix ValueError in Clerks admin if Clerk had no key.
Remove legacy CommandCode
Adds localization base and vendor front page localization
Disallow modifying items already in event.

- This matters if a vendor is allowed to add new items or print their list
  in the event.
Show non-editable items as such.

- Remove inset borders around items that can no longer be edited.
- Don't register click handlers for non-editable items.
Stop outputting html for hidden elements of a printed price tag.

- Generating barcode dataurls can be slow, so not generating them for printed
  items should make pageloads faster and reduce the page size.
Cache barcode dataurls for faster pageloads.

- Significantly reduces the time it takes to reload a page with hundreds of
  barcodes.
Fix few warnings
Adds js-side to localizations
Adds vendor items localization strings.
Adds simple makefile for extracting and compiling messages.
Adds receipt sum for CounterMode
Fix access to checkout API.

Prevent access to checkout API urls when checkout is not activated by
not creating urlpatterns.