Look in organisation for whitelisted domains

At the moment we have to update a YAML file and deploy the change to get
a new domain whitelisted.

We already have a thing for adding new domains – the organisation stuff.

This commit extends the validation to look in the `domains` table on the
API if it can’t find anything in the YAML whitelist.

This has the advantage of:
- not having to deploy code to whitelist a new domain
- forcing us to create new organisations as they come along, so that
  users’ services automatically get allocated to the organisation once
  their domain is whitelisted

app/notify_client/organisations_api_client.py

@@ -1,3 +1,5 @@
+from itertools import chain
+
 from notifications_python_client.errors import HTTPError
 
 from app.notify_client import NotifyAdminAPIClient, _attach_current_user, cache
@@ -8,6 +10,12 @@ class OrganisationsClient(NotifyAdminAPIClient):
     def get_organisations(self):
         return self.get(url='/organisations')
 
+    def get_domains(self):
+        return set(chain.from_iterable(
+            organisation['domains']
+            for organisation in self.get_organisations()
+        ))
+
     def get_organisation(self, org_id):
         return self.get(url='/organisations/{}'.format(org_id))