Ability to share classes / deconflict classes with friends' timetables

[ravern]

Is your feature request related to a problem? Please describe.

I find it difficult to sync up / take the same classes as my friends.

Describe the solution you'd like

The ability to overlay or place my friends' timetables side-by-side, or easily add the courses from their timetable into mine.

Describe alternatives you've considered

Just directly placing 2 NUSMods windows side-by-side, with one of them accessing my friends' timetable through a shared link.

Additional context

Somewhat similar to #141. Reported by Wee Hung through email.