Add JobJobEventsChildrenSummary endpoint

- returns a special view to output the total number of children (and
grandchildren) events for all parents events for a job
value is the number of total children of that event
- intended to be consumed by the UI, as an efficient way to get the
number of children for a particular event
- see api/templates/api/job_job_events_children_summary.md for more info

awx/api/urls/job.py

@@ -10,6 +10,7 @@ from awx.api.views import (
     JobRelaunch,
     JobCreateSchedule,
     JobJobHostSummariesList,
+    JobJobEventsChildrenSummary,
     JobJobEventsList,
     JobActivityStreamList,
     JobStdout,
@@ -27,6 +28,7 @@ urls = [
     re_path(r'^(?P<pk>[0-9]+)/create_schedule/$', JobCreateSchedule.as_view(), name='job_create_schedule'),
     re_path(r'^(?P<pk>[0-9]+)/job_host_summaries/$', JobJobHostSummariesList.as_view(), name='job_job_host_summaries_list'),
     re_path(r'^(?P<pk>[0-9]+)/job_events/$', JobJobEventsList.as_view(), name='job_job_events_list'),
+    re_path(r'^(?P<pk>[0-9]+)/job_events/children_summary/$', JobJobEventsChildrenSummary.as_view(), name='job_job_events_children_summary'),
     re_path(r'^(?P<pk>[0-9]+)/activity_stream/$', JobActivityStreamList.as_view(), name='job_activity_stream_list'),
     re_path(r'^(?P<pk>[0-9]+)/stdout/$', JobStdout.as_view(), name='job_stdout'),
     re_path(r'^(?P<pk>[0-9]+)/notifications/$', JobNotificationsList.as_view(), name='job_notifications_list'),