Discountable amounts by adjustments

[mamhoff]

Summary

This PR changes the new promotion system to calculate discounts entirely via adjustments rather than using in-memory ItemDiscount objects.

The advantage of this approach is that it allows to a) calculate discountable amounts for line items outside of the DiscountOrder loop. This is useful for calculating discounted prices, a feature I have on the roadmap. It specifically covers needs like the one addressed in this commit. This way, we have one system of record, not two.

The other advantage is that we can remove the pretty complex PersistDiscountedOrder code, and just rely on order.save! to do the right thing.

It also dovetails nicely with the work on the in-memory order updater.

Checklist

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[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 89.50%. Comparing base (f8779d6) to head (e3c0bbf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6371      +/-   ##
==========================================
+ Coverage   89.48%   89.50%   +0.01%     
==========================================
  Files         980      982       +2     
  Lines       20438    20485      +47     
==========================================
+ Hits        18289    18335      +46     
- Misses       2149     2150       +1     

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