Implement client-side data validation

Timetable configuration data on the client-side may become invalid for a number of reason

• Lessons get dropped (when there's not enough students in a class)
• Modules disappear
• Lesson timetable get published or updated

etc. etc. Whenever the client receives some data from the API, it should validate its existing timetable configuration data and implement some sanity checks:

• When moduleList.json is received, check that all modules in the timetable still exists
• When modules/<module code>.json is received, check that
  • All lessons are filled
  • All class no. are valid

If these invariants are violated, the app should inform the user in some way (which is why the issue has the user interface label) and take remedial actions, eg. fill up unfilled lesson slots with random timetable configuration.

Trace where the client receives moduleList.json and modules/.json, then inspect how existing timetable configuration is stored and rendered. Add checks for missing modules, unfilled lessons, and invalid class numbers; done means violations are communicated to the user and the timetable receives a remedial action such as filling empty lesson slots.