Add purchase price field to allow net and gross purchase prices

[hanneswernery]

1. Why is this change necessary?

Currently the product and the payload of the order line items have a purchase price field of type float. For multiple reasons a more complex PriceField with gross and net prices as well as currencies would be much more suitable.

2. What does this change do, exactly?

• A new purchase price field purchase_prices of type PriceField was added to the product definition as well as to the order line items (and some corresponding classes).
• The old float purchase price field (and some corresponding functions) are marked as deprecated.
• A migration was added to migrate the deprecated purchase price field to the new one.
• A ApiConverter was added to provide the old purchase price field for the api v1 and the new field for the api v2.
• A database trigger was added to keep the deprecated purchase price field in sync with the new one (updating either will update the other).
• The existing LineItemPurchasePriceRule was removed in favour of LineItemPurchasePriceGrossRule and LineItemPurchasePriceNetRule.
• Existing tests were updated and some new tests were added.
• The administration product view was updated to display the new purchase price field. The layout is open for discussion:

3. Describe each step to reproduce the issue or behaviour.

4. Please link to the relevant issues (if any).

5. Checklist

• [x] I have written tests and verified that they fail without my change
• [x] I have squashed any insignificant commits
• [x] I have written or adjusted the documentation according to my changes
• [x] This change has comments for package types, values, functions, and non-obvious lines of code
• [x] I have read the contribution requirements and fulfil them.

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