Support for Colorado Delivery Fee (flat fee and order-level taxes)

[adammathys]

❗ TODO

• [ ] Add radio buttons for TaxRate#level enum to admin
• [ ] Tweak descriptions for TaxRate#rate and TaxRate#calculator in admin
• [ ] Add specs for order-level taxes
• [ ] Rake task to automatically generate Colorado Delivery Fee stuff

Summary

Add support for the new Colorado Delivery Fee. This is achieved by making two small changes to the existing tax system:

1. New Spree::Calculator::FlatFee calculator. Can be used to return a fixed amount that comes from an associated Spree::TaxRate.
2. Spree::TaxRate#level enum to distinguish between item- and order-level tax rates. Item-level taxes remain the default, since we very rarely if ever want to use order-level tax rates.

These changes in additional to the recent work to support order-level taxes in Spree::OrderTaxation (#4477), makes it possible to configure a tax rate for the Colorado Delivery Fee.

Checklist

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