[Vouchers] Include unfixed amounts and percentage rates

[lin-d-hop]

Description

Extending the first bare minimum iteration of vouchers this next stage will expand the functionality to include:

• Both Percentage and Flat Amount for Voucher Type
• The Amount can be set to anything. For percentage this is between 0 and 100 as a double. For Flat Amount this is a non-negative double.
  
  Screenshot from designs here

The rate will be reflected in the list page and will be calculated accurately at checkout.

Acceptance Criteria & Tests

1. Create voucher codes in the backoffice - using a range of percentages to test edge cases (0, 100 etc) and amounts (more than order total, exactly order total etc)
2. Apply to a checkout flow and see the discount applied
3. If the order total goes below zero when the code is applied it should stay at zero
4. Disregard (at this stage) any case that additional negative fees apply to the order
5. Voucher codes should be able to be used unlimited number of times
6. Voucher codes should apply to any shoppers
7. Voucher codes should not apply in enterprises that have not enabled them. If the enterprise has enabled no voucher code the voucher code box should not be visible at checkout
8. If the voucher code covers the cost of the whole order, just display the full range of payment methods, no clever logic at this stage.
9. Translations as standard
10. Unit tests as standard