Support evaluation subsets in Audition

Audition should accept a query against the predictions table (and a label for the query) to use (and create if necessary) evaluations for a _subset_ of the modeling cohort. Tables required:

- `test_results.subset_evaluations`: same as the evaluations table but with a subset name
- `test_results.entity_subset`: a linking table showing which entities are in associated with each subset at each date

Looking for comments @thcrock @tweddielin @saleiro @rayidghani