Yet another discount scheme -- vouchers or auto-generated, single-use codes.
Registered Vouchers with admin and translation and did a small cleanup of admins for different discounts.
Merge branch 'feature/voucher-discounts' into develop

Conflicts:
	cartridge/shop/admin.py
	cartridge/shop/models.py
Mark voucher codes as used instead of deleteing them.
Merge branch 'feature/voucher-discounts' into develop
PEP8 blank lines and a missing import.
Merge branch 'feature/voucher-discounts' into develop