[Forwardport] save downloadable product links for order items

[swnsma]

Description

Fix save of downloadable links for guest checkout.
During research was found another issue: #19034 which the main reason that problem has appeared.
Events are moved from 'save_commit_after' to 'save_after' to resolve the issue and keep downloadable links save in scope of Sales Order Save transaction.
Forwardport for PR #19036

Fixed Issues (if relevant)

1. Order confirmation email for guest checkout does not include download links #18323: Order confirmation email for guest checkout does not include download links
2. salesInvoiceOrder REST API does not make downloadable products available #19003: salesInvoiceOrder REST API does not make downloadable products available

Manual testing scenarios (*)

Please see issue: #18323

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

Hi @VladimirZaets, thank you for the review.
ENGCOM-3384 has been created to process this Pull Request

[magento-engcom-team]

Hi @swnsma. 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.