Do not cascade delete job_events

* want to drop job_event _partitions_
  .. instead of having the job events associated with a job
     automatically get cleaned up for us

awx/main/migrations/0130_event_partitions.py

@@ -114,4 +114,9 @@ class Migration(migrations.Migration):
             name='job_created',
             field=models.DateTimeField(null=True, editable=False),
         ),
+        migrations.AlterField(
+            model_name='jobevent',
+            name='job',
+            field=models.ForeignKey(editable=False, null=True, on_delete=models.deletion.SET_NULL, related_name='job_events', to='main.Job'),
+        ),
     ]

awx/main/models/events.py

@@ -476,7 +476,8 @@ class JobEvent(BasePlaybookEvent):
     job = models.ForeignKey(
         'Job',
         related_name='job_events',
-        on_delete=models.CASCADE,
+        null=True,
+        on_delete=models.SET_NULL,
         editable=False,
     )
     host = models.ForeignKey(