Multi class score

[mike-kaimika]

This PR allows for more than one class score data directory per dataset, with some enhancements to specify the model users for the class score

1. Datasets can now have multiple class score directories, each with a unique model identifier
2. The bin page lists out all class scores, including the model
3. Endpoints handle a ?model= parameter to choose a specific model
4. When no model is specified, the default sorting of the class score directories will first be by a new is_class_score_default setting, and then by most recent
5. .csv and .mat exports will include the model name. These changes also remove the version portion of the filename, which was not working all the time
6. Fixes issues with the existing path check during validation when saving a data directory

@joefutrelle There were a lot more edge cases than originally expected, so I'd appreciate second set of eyes on this to make sure nothing was missed from what you were expecting

New admin settings:

New display of multiple class scores, including model (if present)