make add_rate_to_jobs singular

there was no reason for it to operate on a whole list at a time

app/statistics_utils.py

@@ -81,8 +81,8 @@ def get_failure_rate_for_job(job):
     )
 
 
-def add_rate_to_jobs(jobs):
-    return [dict(
+def add_rate_to_job(job):
+    return dict(
         failure_rate=(get_failure_rate_for_job(job)) * 100,
         **job
-    ) for job in jobs]
+    )