Modulplaner OpenAPI

OpenAPI documentation of the modulplaner API

Improvements

Rename /data/ to /api/
Rename teachers to lecturers
Rename id fields to pk to avoid many naming collisions in programming languages
Move any binary files away from the API (xlsx, PDF, …)
The blockclasses should be part of the {version} api, not the {semester}
Change some German names to English (klassen.pdf, …)
Decide on Blockclass vs Blockmodule