Add descriptions to push notification categories

Gijs Hendriksen
2018-09-19T20:17:56+02:00

Add descriptions to notification categories

### Desired behaviour
As described in ThaliApp#75, the ThaliApp settings screen can now only display the push notifications category names. However, some of these names don't make it clear what kind of push notifications are sent through in this category. Thus, it would be useful to add an explanation of each of the categories.