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/invite_api_client.py

@@ -44,8 +44,8 @@ class InviteApiClient(NotifyAdminAPIClient):
         self.post(url='/service/{0}/invite/{1}'.format(service_id, invited_user_id),
                   data=data)
 
-    @cache.delete('service')
-    @cache.delete('user', key_from_args=[1])
+    @cache.delete('service', 'service_id')
+    @cache.delete('user', 'invited_user_id')
     def accept_invite(self, service_id, invited_user_id):
         data = {'status': 'accepted'}
         self.post(url='/service/{0}/invite/{1}'.format(service_id, invited_user_id),