feat: include course_key in final output models (FC-0033)

[SoryRawyer]

Including course_key in the models used by Superset will allow us to make better use of our primary keys, thereby improving query performance and stability. The problem interaction models already had course_key and problem_id in the models so all that was needed was to bring the documentation in line with the implementation.

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[bmtcril]

Released as v2.1