Get info about organisations from database table

This is the first step of replacing the `domains.yml` file.

In order to replicate the same functionality we get from the
`domains.yml` file and its associated code this commit adds a
`Organisation` model. This model copies a lot of methods from the
`AgreementInfo` class which wrapped the `domains.yml` file.

It factors out some stuff that would otherwise be duplicated between the
`Organisation` and `Service` model, in such a way that could be reused
for making other models in the future.

This commit doesn’t change other parts of the code to make use of this
new model yet – that will come in subsequent commits.

app/notify_client/organisations_api_client.py

@@ -1,3 +1,5 @@
+from notifications_python_client.errors import HTTPError
+
 from app.notify_client import NotifyAdminAPIClient, _attach_current_user, cache
 
 
@@ -9,6 +11,16 @@ class OrganisationsClient(NotifyAdminAPIClient):
     def get_organisation(self, org_id):
         return self.get(url='/organisations/{}'.format(org_id))
 
+    def get_organisation_by_domain(self, domain):
+        try:
+            return self.get(
+                url='/organisations/by-domain?domain={}'.format(domain),
+            )
+        except HTTPError as error:
+            if error.status_code == 404:
+                return None
+            raise error
+
     def create_organisation(self, name):
         data = {
             "name": name