Improve handling of different types

The following discussion from !47 (merged) should be addressed:

@nik started a discussion:

> Things like these should never be "magic strings". An Enumeration might be a good choice.
> 
> This needs to be changed in existing code, which is out of scope of this MR, but please make sure to track this issue before resolving this thread.

• Substitution types (cancellation, cancellation_for_teachers, substitution)
• Timetable types (teacher, group, room)