Differentiate permissions for timetables into group/teacher/room permissions

The following discussion from !301 (merged) should be addressed:

• @yuha started a discussion:

  FIXME: Differentiate permissions for timetables into group/teacher/room permissions (like it was before)

changed title from Follow-up from "Draft: New data model based on calendar events" to Differentiate permissions for timetables into group/teacher/room permissions

added part::backend type::bug labels

changed milestone to /AlekSIS%"[NLnet #1 (closed)] Fix existing bugs with new timetable"