fix: sidebar completion for completable XBlocks with children

[Agrendalath]

Description

It is possible to create a completable XBlock with children. An example is the Library Content Block with the MARK_LIBRARY_CONTENT_BLOCK_COMPLETE_ON_VIEW feature toggle. The sidebar should use the same mechanism as the BlockCompletionTransformer and the edx-completion library. It means that we should treat:

1. An aggregator XBlock as completed only when all its children are completed.
2. A completable XBlock as completed when it is directly marked as completed (without checking the completion of its children).

Testing instructions

1. Do not check out the branch from this PR.
2. Set settings.FEATURES['MARK_LIBRARY_CONTENT_BLOCK_COMPLETE_ON_VIEW'] = True.
3. Create a new library with a Problem XBlock.
4. Create a new subsection with two units - the first with this new library and the second with any other XBlock. Publish these blocks.
5. Visit the Library Content Block in the LMS. Do not submit the problem - only wait a few seconds for the publish_completion request to be sent (you can check it, e.g., with the Network tab).
6. Click the button to navigate to the second unit (without refreshing the page). This will trigger the get_completion request, which retrieves the completion data from the completion service (from the edx-completion).
7. Refresh the page. See that the completion checkmark for the first unit is now gone.
8. Check out the branch from this PR.
9. Refresh the page. See that the completion checkmark for the first unit is now displayed correctly.

Deadline

"None"

Private-ref: BB-8469 (temporary)

[openedx-webhooks]

Thanks for the pull request, @Agrendalath!

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