feat: add models for video engagement dropoff (FC-0051)

[SoryRawyer]

This adds one user-facing model (fact_video_engagement) and one internal model (int_videos_per_subsection) in order to support a chart that displays how many learners engaged with videos within a specific section or subsection. The macro items_per_subsection is meant to make it easier to create similar intermediate models for other item types (like problems or pages).

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[SoryRawyer]

@Ian2012 @bmtcril do either of you have suggestions for resolving this build error? Is this similar to the other issues I've had, where I'm lacking permissions to run certain steps?

[bmtcril]

Yeah that error is related to not having write on the repo, I haven't found a way around it yet so we should just consider those non-blocking for now.

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