Managing timetable information for groups

Timetable information for groups as a new optional data is introduced by backend-core#205 (closed).

Required frontend features:

• Instructors should be able to edit this information for a group.
• Timetable information (if provided) should be visible in the detailed view both for students and instructors.
• Timetable information (if provided) should be visible in the left sidebar for each group.
• Currently only a default sorting (course name, group number) is possible in the left sidebar. As an alternative, sorting by timetable information should be possible as well.