Add sent message overview to the push notifications API

Gijs Hendriksen
2019-10-24T19:17:12+02:00

Needed for ThaliApp#99

### Short description

Adds a new route to the push notifications API, where one can view all push messages sent to them.

### Steps to use this feature

1. Head to `/api/v1/notifications/`.
2. See list of messages sent through push notifications.
3. View one notification at `/api/v1/notifications/[pk]/` (even though it does not provide any more information).