Absence reason as predefined list for absence categories

Absences can have a comment to tell the reason if it is known. While this is fine to add specific details it is not usefull to be displayed in a list and it does not help cluster the absences into groups. 

A list of absence categories shall be configurable in the admin section. If such absence categories are created, the categories will show up in the roll-call dialog as a pulldown menu and also in the new absence notice dialog.

The absence categories are optional, if none defined the UI is as it is today.