Modify stats endpoints to return unique DCC/category ids.

[jonathancrabtree]

Currently the stats endpoints only return the DCC abbreviation, which can cause problems if there's a discrepancy between the DCC abbreviation in the release catalog and the DCC abbreviation in the registry (see nih-cfde/dashboard#96). More generally, the DCCGrouping return type in the current API should be modified (if possible) to return unique ids for all of the categories (e.g., data_type, assay, species, and anatomy.) Without the id the UI doesn't have a reliable way to match these terms with the user's favorites.

[ACharbonneau]

is this still an issue we need to solve?