Add search functionality to the events API

Gijs Hendriksen
2019-11-06T21:35:44+01:00

Needed for ThaliApp#92.

### Short description
Adds the ability to search for events in the events API.

### Steps to use this feature

1. Go to `/api/v1/events/?search=[query]`.
2. Notice the events list is filtered on the query terms, and past events are shown, too.