Allow category to be source or target when matching in Sales and Discounts

[nfourtythree]

At the moment they both require the category to be a field on the product. There are use cases where they might be in a related products field on the Category.

Current matches (https://github.com/craftcms/commerce/blob/develop/src/services/Discounts.php#L377) and (https://github.com/craftcms/commerce/blob/develop/src/services/Sales.php#L363).

We should also look to potentially combine these checks to reduce the code replication.

This will likely be a breaking change for some users as it might discount/apply sales to unexpected products.

[nfourtythree]

Fixed in 4a568ea

[nfourtythree]

Due to this being a significant breaking change, this functionality is being moved to the 3.0 release

[nfourtythree]

This functionality is now on the 3.0 development branch and will be included in the next 3.0 pre-release.