Allow event detail viewing in the API

Closes ThaliApp#62

Previous behaviour

When opening a past event in the app, the app indicates the event cannot be loaded.

Steps to reproduce:

1. Open the calendar on the website.
2. Click on an event that has already happened.
3. Let deeplinking take you to the app.
4. Notice that the event cannot be loaded.

New behaviour

When opening a past event in the app, the event is loaded in the app.

Steps to validate that it works:

1. Open the calendar on the website.
2. Click on an event that has already happened.
3. Let deeplinking take you to the app.
4. Notice that the event is loaded.