Add a page to set organisation and branding option

Platform admin only.

Adds radio buttons to choose one of:
- three hard-coded branding options
- organisations from a list provided by the API

app/notify_client/organisations_client.py

@@ -0,0 +1,19 @@
+from notifications_python_client.base import BaseAPIClient
+
+
+class OrganisationsClient(BaseAPIClient):
+
+    def __init__(self, base_url=None, client_id=None, secret=None):
+        super(self.__class__, self).__init__(
+            base_url=base_url or 'base_url',
+            client_id=client_id or 'client_id',
+            secret=secret or 'secret'
+        )
+
+    def init_app(self, app):
+        self.base_url = app.config['API_HOST_NAME']
+        self.client_id = app.config['ADMIN_CLIENT_USER_NAME']
+        self.secret = app.config['ADMIN_CLIENT_SECRET']
+
+    def get_organisations(self):
+        return self.get(url='/organisation')['organisations']