Check if any jobs exist before querying jobs

At the moment the dashboard does two API calls to find out if a service
has:

1. Scheduled jobs
2. Normal jobs

API calls are slow because they are synchronous, go over the network and
touch the database. We can’t cache these API calls because:
- a scheduled job could become a normal job at any time
- the statistics on a normal job are constantly updating

However there are plenty of services which don’t have any jobs, and
probably never will. And finding out if a service has any jobs is
reliably cacheable (because as soon as a service creates its first job
it has some jobs).

So this commit:
- refactors the way we get scheduled/normal jobs into the job_api_client
  to make the view a bit slimmer
- makes an additional, Redis-wrapped call to find out if any jobs exist
  before trying to get the jobs

This should result in a speedup on the dashboard, and can be used in the
future if there’s anywhere else we want to show or hide something
depending on whether a service has created any jobs (I have some ideas).

app/main/views/dashboard.py

@@ -275,21 +275,18 @@ def aggregate_usage(template_statistics, sort_key='count'):
 
 
 def get_dashboard_partials(service_id):
-    # all but scheduled and cancelled
-    statuses_to_display = job_api_client.JOB_STATUSES - {'scheduled', 'cancelled'}
-
     template_statistics = aggregate_usage(
         template_statistics_client.get_template_statistics_for_service(service_id, limit_days=7)
     )
 
-    scheduled_jobs = sorted(
-        job_api_client.get_jobs(service_id, statuses=['scheduled'])['data'],
-        key=lambda job: job['scheduled_for']
-    )
-    immediate_jobs = [
-        add_rate_to_job(job)
-        for job in job_api_client.get_jobs(service_id, limit_days=7, statuses=statuses_to_display)['data']
-    ]
+    scheduled_jobs, immediate_jobs = [], []
+    if job_api_client.has_jobs(service_id):
+        scheduled_jobs = job_api_client.get_scheduled_jobs(service_id)
+        immediate_jobs = [
+            add_rate_to_job(job)
+            for job in job_api_client.get_immediate_jobs(service_id)
+        ]
+
     stats = service_api_client.get_service_statistics(service_id, today_only=False)
     column_width, max_notifiction_count = get_column_properties(
         number_of_columns=(

app/main/views/jobs.py

@@ -42,9 +42,7 @@ from app.utils import (
 @user_has_permissions('view_activity')
 def view_jobs(service_id):
     page = int(request.args.get('page', 1))
-    # all but scheduled and cancelled
-    statuses_to_display = job_api_client.JOB_STATUSES - {'scheduled', 'cancelled'}
-    jobs_response = job_api_client.get_jobs(service_id, statuses=statuses_to_display, page=page)
+    jobs_response = job_api_client.get_page_of_jobs(service_id, page=page)
     jobs = [
         add_rate_to_job(job) for job in jobs_response['data']
     ]