Create Spree::SimpleOrderContents

[mamhoff]

Summary

Spree::OrderContents interacts heavily with the current promotion system. This adds a new Spree::SimpleOrderContents class that we can use once all the core functionality and specs for the legacy promotion system are migrated to solidus_legacy_promotions.

We're keeping Spree::OrderContents though, with the intention of moving it to solidus_legacy_promotions in the future.

Extracted from #5634

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