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/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']
     ]