Add Custom Modules to Timetable

[seanlowjk]

Context

Resolves #2040

The idea is to implement custom modules for students who wish
to add modules that are not part of the NUS curriculum (DYOMs)
or TA classes. (Especially for SoC TAs)

Implementation

Timetable.-.NUSMods.-.Google.Chrome.2021-12-15.17-38-22.mp4

With guidance from @chrisgzf , we have developed a solution which allows
users to add their custom modules.

This is done via CustomLessons which are not readonly. As a result, we can
incorporate these custom lessons into the TimetablesState to be stored for future use.

Other Information

1. Once this is integrated, will we need to integrate this feature as well with
   regards to the export feature?
2. My engineering decisions have been briefly mentioned above. Do let me know
   if there is room for improvement.

TODOs

• Add Component and Logic Tests