Added canceled_on field to unified_jobs model

- When a job is canceled, the canceled_on field will populate with date/time

awx/main/migrations/0101_v370_unifiedjob_canceled.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.2.4 on 2019-11-25 20:53
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0100_v370_projectupdate_job_tags'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='unifiedjob',
+            name='canceled_on',
+            field=models.DateTimeField(db_index=True, default=None, editable=False, help_text='The date and time when the cancel request was sent.', null=True),
+        ),
+    ]