added a category field and added it to the admin with filtering

Luko van der Maas
2017-12-27T23:16:37+01:00

~feature

Closes #537

### Short description
Added a few categories for push notification messages:
* General (Default)
* Pizza
* Events
* Newsletter
* Sponsored messages
* Photo's
* Board

### Steps to use this feature

1. In the admin page for push notification messages choose a category.
2. Filter the messages on a certain category