[Forwardport] Make sure all linked products (related, upsells, crosss…

[hostep]

…ells) show up in the backend grids and in the correct order. Fixes #13720

(cherry picked from commit a3f1c38)

Description

As requested in #17885 (comment), this is a forward port of #17885 for the 2.3 release line.
See original PR for full description.

Fixed Issues (if relevant)

1. Only 2 related products are showing in backend . #13720: Only 2 related products are showing in backend
2. Import related products issue #14050: Import related products issue

Manual testing scenarios

1. Have a product with 5 to 10 related products
2. Go into the database and look at the table catalog_product_link_attribute_int
3. All positions (value column, for product_link_attribute_id == 1) should be unique if you created those related products through the backend
4. To simulate a product import where the positions can end up being non-unique, run the following query: UPDATE catalog_product_link_attribute_int SET value = 1 WHERE product_link_attribute_id = 1;
5. Take a look at the product in the adminhtml and open the Related Products, Up-Sells, and Cross-Sells section.
6. Without this fix, you'll only see 2 products, not the full list, with this fix all products show up in the correct sort order.
7. Also check the frontend, before and after the fix, the sort order will be the same over there, so frontend sort order isn't affected by this change.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @hostep. Thank you for your contribution
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[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-3035 has been created to process this Pull Request

[magento-engcom-team]

Hi @hostep. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.