Always create TCCs and use them as single uniform interface during planning

[Course]  (SchoolTerm via Groups)
   ↓
 [TCC]    (ValidityRange)
   ↓
[Lesson]  (Concrete Lesson)

Plan: Creating a ValidityRange creates a TCC for each course in its term. (TCC copies lesson-quota from course)

Timetables are then planned with TCCs (not Courses & TCCs anymore).

Course: name subject teachers groups lesson-quota default-room
TCC: course validity-range lesson-quota teachers
Validity-Range: school-term name date-start date-end status
Lesson: slot-start slot-end recurrence rooms teachers subject