Make stats endpoint a little less verbose

@joeadams04 requested some changes to the stats endpoint. I propose:

{
    "assetCounts": {
        "all": {
            "total": 1234,
            "completed": 1234,
            "withPersonalData": 1234
        },
        "byInstitution": {
            "INSTA": {
                "total": 123,
                "completed": 123,
                "withPersonalData": 123
            }
            // ... etc
        }
    }
}

@joeadams04, does that seem OK?