Score Thresholds from Contributors

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We should add functionality to ingest score thresholds and model fits from pillar project contributors, such as those here: https://github.com/Dzeiberg/mave_calibration.

We should add relevant visualizations for these thresholds, provide a framework for their data model (which ideally generalizes well to future thresholds we may include), and provide a mechanism for including such thresholds on future variant level pages.