Rewrite cache decorator to reference args by name

`@cache.delete('user', 'user_id')` is easier to read and understand than
`@cache.delete('user', key_from_args=[1])`. This will become even more
apparent if we have to start doing stuff like `key_from_args=[1, 5]`,
which is a lot more opaque than just saying
`'service_id', 'template_id'`.

It does make the implementation a bit more complex, but I’m not too
worried about that because:
- the tests are solid
- it’s nicely encapsulated

app/notify_client/organisations_api_client.py

@@ -27,7 +27,7 @@ class OrganisationsClient(NotifyAdminAPIClient):
     def get_service_organisation(self, service_id):
         return self.get(url="/service/{}/organisation".format(service_id))
 
-    @cache.delete('service')
+    @cache.delete('service', 'service_id')
     def update_service_organisation(self, service_id, org_id):
         data = {
             'service_id': service_id