use new letter branding instead of dvla organisation id

new code is copied stylistically from the email branding patterns.
Instead of `service.dvla_organisation`, there's now
`service.letter_branding` and `service.letter_branding_id`. However,
unlike email branding we're not currently showing a preview of the
logo. That can come later when we work out how we want to do it.

app/notify_client/letter_branding_client.py

@@ -1,10 +1,24 @@
-from app.notify_client import NotifyAdminAPIClient
+from app.notify_client import NotifyAdminAPIClient, cache
 
 
 class LetterBrandingClient(NotifyAdminAPIClient):
 
-    def get_letter_branding(self):
-        return self.get(url='/dvla_organisations')
+    @cache.set('letter_branding-{branding_id}')
+    def get_letter_branding(self, branding_id):
+        return self.get(url='/letter-branding/{}'.format(branding_id))
+
+    @cache.set('letter_branding')
+    def get_all_letter_branding(self, sort_key=None):
+        brandings = self.get(url='/letter-branding')
+        if sort_key and sort_key in brandings[0]:
+            brandings.sort(key=lambda branding: branding[sort_key].lower())
+        return brandings
+
+    def get_letter_branding_id_for_domain(self, domain):
+        for branding in self.get_all_letter_branding():
+            if domain and branding.get('domain') == domain:
+                return branding['id']
+        return None
 
     def create_letter_branding(self, filename, name, domain):
         data = {

app/notify_client/service_api_client.py

@@ -75,6 +75,7 @@ class ServiceAPIClient(NotifyAdminAPIClient):
             'sms_sender',
             'created_by',
             'branding',
+            'letter_branding',
             'email_branding',
             'letter_contact_block',
             'dvla_organisation',