Add management of absence reasons

Custom absence reasons have to be a. introduced in the data model and b. be manageable in the frontend. A mock-up for this is still missing and will be added when #255 (closed) is completed.

changed milestone to /AlekSIS%"[NLnet #3] Implement support for absences"

added part::backend part::frontend type::feature labels

this is already covered in AlekSIS-App-Kolego and thus no additional work is required (@hansegucker ?)

closed