Split App component into multiple smaller components

[diogotcorreia]

Also changes them to be functional components instead of class components

A few behaviors that I've changed:

• When deleting a timetable that's not the active one, it now stays in that timetable instead of going to the first one
• Deleting a timetable now shows an alert on success

Things under the hood that have been changed:

• ShiftRef was deleted (didn't seem to be doing anything)
• Derived state is no longer calculated on state update, but on render (and memoized). This makes it much simpler to update state
• Domain classes Timetable, Course and Shift are now fully immutable
• CourseUpdate class was deleted (was just adding unneeded complexity)
• I've tried to make sure there is only one source of truth across the app, so we don't have outdated data spread across components

[diogotcorreia]

Almost done, just need to fix a few things I broke in the process!

• Change NewTimetable to functional component
• Add colors to excel export
• Add colors to image export
• Fix duplicate schedule button
• Fix shift classes button
• Fix console errors
• Replace withStyles with useStyles on App.tsx
• Fix // TODOs added

[diogotcorreia]

I think this is done, and it has feature parity with the production version.
I've updated the top comment with some information.

I have a few more things that I want to change, but I'll open issues instead and do them on another PR, since this is huge already.
Consider squashing this instead of merging.

[joaocmd]

@all-contributors please add @diogotcorreia for code

[allcontributors]

@joaocmd

I've put up a pull request to add @diogotcorreia! 🎉