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/notify_client/job_api_client.py

@@ -1,6 +1,6 @@
 from collections import defaultdict
 
-from app.notify_client import NotifyAdminAPIClient, _attach_current_user
+from app.notify_client import NotifyAdminAPIClient, _attach_current_user, cache
 
 
 class JobApiClient(NotifyAdminAPIClient):
@@ -16,6 +16,8 @@ class JobApiClient(NotifyAdminAPIClient):
         'sent to dvla'
     }
 
+    NORMAL_JOB_STATUSES = JOB_STATUSES - {'scheduled', 'cancelled'}
+
     def __init__(self):
         super().__init__("a" * 73, "b")
 
@@ -60,8 +62,38 @@ class JobApiClient(NotifyAdminAPIClient):
 
         return jobs
 
+    def get_page_of_jobs(self, service_id, page):
+        return self.get_jobs(
+            service_id,
+            statuses=self.NORMAL_JOB_STATUSES,
+            page=page,
+        )
+
+    def get_immediate_jobs(self, service_id):
+        return self.get_jobs(
+            service_id,
+            limit_days=7,
+            statuses=self.NORMAL_JOB_STATUSES,
+        )['data']
+
+    def get_scheduled_jobs(self, service_id):
+        return sorted(
+            self.get_jobs(service_id, statuses=['scheduled'])['data'],
+            key=lambda job: job['scheduled_for']
+        )
+
+    @cache.set('has_jobs-{service_id}')
+    def has_jobs(self, service_id):
+        return bool(self.get_jobs(service_id)['data'])
+
     def create_job(self, job_id, service_id, scheduled_for=None):
 
+        self.redis_client.set(
+            'has_jobs-{}'.format(service_id),
+            True,
+            ex=cache.TTL,
+        )
+
         data = {"id": job_id}
 
         if scheduled_for:
@@ -78,6 +110,7 @@ class JobApiClient(NotifyAdminAPIClient):
 
         return job
 
+    @cache.delete('has_jobs-{service_id}')
     def cancel_job(self, service_id, job_id):
 
         job = self.post(