Add API client method to count templates

When users request to go live we check stuff like:
- if they’ve added templates
- if they have email templates (then we can check their reply to
  address)

This commit adds a method to do this programatically rather than
manually.

We _could_ do this in SQL, but for page that’s used intermittently it
doesn’t feel worth the work/optimisation (and the client method is at
least in place now if we do ever need to lean on this code more
heavily).

app/notify_client/service_api_client.py

@@ -226,6 +226,16 @@ class ServiceAPIClient(NotifyAdminAPIClient):
             service_id=service_id)
         return self.get(endpoint, *params)
 
+    def count_service_templates(self, service_id, template_type=None):
+        return len([
+            template for template in
+            self.get_service_templates(service_id)['data']
+            if (
+                not template_type
+                or template['template_type'] == template_type
+            )
+        ])
+
     def delete_service_template(self, service_id, template_id):
         """
         Set a service template's archived flag to True