feat(timetable): restore customizable timetables

[zwliew]

Context

This brings back PRs #3420 and #3434, which were reverted as part of PR #3435.

This re-resolves issue #3404.

Implementation

The original schema migration is faulty, so I added a minor fix for that.

Other Information

TODOs:

1. Some tests would be good.
2. Hide in exam schedule.
3. Add a tally for how many course units are customized.
4. Deconflict activeLessons and customiseModule in the app state.

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[codecov[bot]]

Codecov Report

Patch coverage: 45.45% and project coverage change: -0.21% :warning:

Comparison is base (71f0fbd) 53.22% compared to head (551c8e1) 53.01%. Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3483      +/-   ##
==========================================
- Coverage   53.22%   53.01%   -0.21%     
==========================================
  Files         271      271              
  Lines        5860     5964     +104     
  Branches     1398     1436      +38     
==========================================
+ Hits         3119     3162      +43     
- Misses       2741     2802      +61     

Files Changed	Coverage Δ
website/src/reducers/app.ts	80.64% <0.00%> (-5.57%)	:arrow_down:
website/src/types/reducers.ts	100.00% <ø> (ø)
...c/views/components/module-info/LessonTimetable.tsx	85.71% <ø> (ø)
website/src/views/settings/BetaToggle.tsx	0.00% <0.00%> (ø)
website/src/views/settings/SettingsContainer.tsx	0.00% <ø> (ø)
website/src/views/tetris/TetrisGame.tsx	0.00% <ø> (ø)
website/src/views/timetable/Timetable.tsx	93.54% <ø> (ø)
website/src/views/timetable/TimetableDay.tsx	55.00% <ø> (ø)
website/src/views/timetable/TimetableRow.tsx	94.73% <ø> (ø)
website/src/views/venues/VenueDetails.tsx	9.09% <ø> (ø)
... and 7 more

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