* refactor!: use String field instead of Dict field to store top_level_downstream_parent_key
Since this is a new field no production instance should have this field
yet. Developers need to delete their old courses as this change will
raise error in all course pages.
* chore: add `top_level_parent` field in ComponentLink and ContainerLink admin
* refactor: use ":" as separator
* refactor: block key parsing and tests