Add pizza order button to the event detail

Gijs Hendriksen
2018-01-28T20:24:21+01:00

Closes #34

### Short description
This adds a button to order pizza to the event detail of events where pizza can be ordered.

![pizza](/uploads/bb9e46f6dc9f22275321df1f1ef93611/pizza.png)

### Steps to use this feature

1. Open an event for which you can order pizza.
2. Press the "Bestel" button.
3. ..?
4. Pizza!