- Adds the `top_level_parent_usage_key` to the `EntityLinkBase`
- This field is used to save the top-level parent of a component or container when it is imported into a course. Example: A unit with components imported into a course. The unit is the top-level parent of the components.
- Updates the `DownstreamListView` to return the top-level parents instead of downstream child, if this parent exists.
- Each time containers with children were synchronized, a new downstream block was created for each child instead of updating the existing one. This occurred because the `upstream_key` was incorrectly validated as an `Opaquekey` against a list of key strings. This was fixed by converting the `upstream_key` to a string before the verification. (see 34cd5a4781 and 29647831dc)
- Which edX user roles will this change impact? "Course Author", "Developer".
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