[3.0] Move Subject from Chronos to Core
I would like to transfer Subject to Core because there are situations like grade management that should be also possible without a timetable. Maybe we should also structure the subject concept better with parent subjects (like English or Maths) and course/child subjects (like Advanced Maths or English 1). That would prevent some problems discovered while developing the model for AlekSIS-App-Vidnesbyrd.