Add categories to push notifications

One-sentence description

Add categories to push notifications.

Desired behaviour

To make filtering of push notifications possible for users we would like to have categories that we can assign certain messages to so that users can choose to not receive those messages. For example:

Pizza reminders
Event registration (registration start)
Event information (event start and location)
Newsletters (get a notification when a new newsletter is available and open in browser)
Sponsored messages
New photos available
Messages from the board

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