feat: adds models for problem interaction reports (FC-0024)

[SoryRawyer]

Models should support the following reports listed in #4 :

• problem difficulty (via learner_problem_summary)
• answer distributions per problem (via problem_responses)
• problem submission counts (also via problem_responses)

These models have been tested with the force_primary_key=1 setting to ensure they make proper use of indexes.

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[bmtcril]

I think we're going to need to do some fiddling as the visualizations come online but this is looking good. We're also going to need to start joining in the block names from the event_sink schema, but I need to figure out how to make fake ones from the generated fake data first.

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