Include Tower configurations into activity stream

Relates #7386 of ansible-tower.

Due to the uniqueness of Tower configuration datastore model, it is not
fully compatible with activity stream workflow. This PR introduced
setting field for activitystream model along with other changes to make
Tower configuration a special case for activity streams.

Signed-off-by: Aaron Tan <jangsutsr@gmail.com>

awx/api/serializers.py

@@ -3797,6 +3797,11 @@ class ActivityStreamSerializer(BaseSerializer):
 
                     if fk == 'schedule':
                         rel['unified_job_template'] = thisItem.unified_job_template.get_absolute_url(self.context.get('request'))
+        if obj.setting and obj.setting.get('category', None):
+            rel['setting'] = self.reverse(
+                'api:setting_singleton_detail',
+                kwargs={'category_slug': obj.setting['category']}
+            )
         return rel
 
     def _get_rel(self, obj, fk):
@@ -3848,6 +3853,8 @@ class ActivityStreamSerializer(BaseSerializer):
                                            username = obj.actor.username,
                                            first_name = obj.actor.first_name,
                                            last_name = obj.actor.last_name)
+        if obj.setting:
+            summary_fields['setting'] = [obj.setting]
         return summary_fields