Openedx-completion-aggregator

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* Add Lilac support.

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* Add Koa support.
* Upgrade Python to 3.8.

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* Fix handling of invalid keys.

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* Fix `all` option in `reaggregate_course`.

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* Pass `user.username` to Celery task instead of `user`.
* Convert `course_key` string to `CourseKey` in `reaggregate_course`.

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* Add locking mechanism to batch operations.
* Replace `course_key` with `course` in `reaggregate_course` management command.