feat: add model for at-risk learners (FC-0051)

[SoryRawyer]

This change adds a new model, dim_at_risk_learners, which aims to collect a list of learners who meet the criteria for being at risk of dropping out of a given course.

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[SoryRawyer]

@bmtcril It's ready, I was just unsure about the logic for determining at-risk users so I figured I'd leave it in draft until that was cleared up.

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