Lessons Configs are Not Preserved When Faculty Modifies Timetable

[zehata]

Describe the bug

When the lesson timetable for a module is changed by the faculty, the lesson indices in the config points to the wrong lessons.

To Reproduce

WIP

Expected behavior

A better implementation of the validation logic should be able to compare the cached timetable before the module update and the timetable after the change, in order to recover the config that the user has created before the faculty changed the timetable.

Additional context

This is caused by #4225

[HollaG]

by the way, this affects the share URL as well - I tried to open a share URL sent by my friend a while ago, and due to faculty changes, the imported timetable ends up missing some lessons.
https://nusmods.com/timetable/sem-2/share?BN2112=LEC:(0);LAB:(1)&CDE2300=LEC:(0)&CE2407B=LEC:(0,1);TUT:(2)&CS1010E=SEC:(0);TUT:(28)&EG1311=LAB:(1);LEC:(14)&MA1511=TUT:(11);LEC:(15)&RVSS1002=SEC:(0)

importing this and re-sharing / inspecting reveals that the URL changes to
https://nusmods.com/timetable/sem-2/share?BN2112=LEC:(0);LAB:(1)&CDE2300=LEC:(0)&CE2407B=LEC:(0,1);TUT:(2)&CS1010E=SEC:();TUT:(28)&EG1311=LAB:(1);LEC:()&MA1511=TUT:(11);LEC:()&RVSS1002=SEC:(0)

Note the invalid formats where there are empty tuples. This directly results to nothing being displayed on the timetable.

The user might be unaware of this, for example a use case:

1. User creates timetable on home computer during the holidays, and then sends a saved link to self
2. When school starts, user imports the saved link on a laptop
3. No alerts about changed timetable.

While a shift in timing is more visible to a user who simply follows their timetable, a complete disappearance is more severe as (esp for freshmen) they might not realise that they are missing a lesson.

This should really result in an obvious warning message, prompting them to re-add the whole module.

[zehata]

@HollaG Agreed