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).
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