Refactor to avoid redefinition of org types

We’re defining the list of org types in a few different places. This
makes it more likely we’ll forget to update one of these places, thereby
introducing a bug.

This commit moves the definition to be on the organisation model, which
feels like a sensible enough place for it.

app/models/organisation.py

@@ -7,6 +7,17 @@ from app.notify_client.organisations_api_client import organisations_client
 
 class Organisation(JSONModel):
 
+    TYPES = (
+        ('central', 'Central government'),
+        ('local', 'Local government'),
+        ('nhs_central', 'NHS – central government agency or public body'),
+        ('nhs_local', 'NHS Trust or Clinical Commissioning Group'),
+        ('nhs_local', 'GP practice'),
+        ('emergency_service', 'Emergency service'),
+        ('school_or_college', 'School or college'),
+        ('other', 'Other'),
+    )
+
     ALLOWED_PROPERTIES = {
         'id',
         'name',