Cache known domains in Redis

The domains lookup is a bit slow because it’s serialising all the
organisations in the database. Since we’re putting this in the sign up
flow it should feel snappy, so lets cache just the domain bit of it in
Redis.

app/notify_client/organisations_api_client.py

@@ -10,8 +10,9 @@ class OrganisationsClient(NotifyAdminAPIClient):
     def get_organisations(self):
         return self.get(url='/organisations')
 
+    @cache.set('domains')
     def get_domains(self):
-        return set(chain.from_iterable(
+        return list(chain.from_iterable(
             organisation['domains']
             for organisation in self.get_organisations()
         ))
@@ -35,6 +36,7 @@ class OrganisationsClient(NotifyAdminAPIClient):
         }
         return self.post(url="/organisations", data=data)
 
+    @cache.delete('domains')
     def update_organisation(self, org_id, **kwargs):
         return self.post(url="/organisations/{}".format(org_id), data=kwargs)