Chronos parent menu point is shown regardless of permissions (even though a user is not allowed to access any timetables)

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

• @yuha started a discussion:

  FIXME: Chronos parent menu point is shown regardless of permissions (even though a user is not allowed to access any timetables).

changed title from Follow-up from "Draft: New data model based on calendar events" to Chronos parent menu point is shown regardless of permissions (even though a user is not allowed to access any timetables)

added part::frontend type::bug labels

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