Making Courses in Semesters Sortable

[ZzRanger]

Motivation
Writing this to record our attempt at making courses sortable within the semester plan.

Why make semesters sortable
Reason was twofold:

1. The beta version w/ react-beautiful-dnd had sortable courses
2. We assumed that users would want to reorder their courses within each semester

What we tried

1. Since we're using dnd-kit, we attempted to use the useSortable hook to make it sortable
2. The code for useSortable kinda long
3. We got it to work, but it's very laggy on certain people's computers

See #407 for what we tried

Why we abandoned it

1. We realized that the problem is that courses aren't logically sorted at all, not that users can't sort them
2. We decided making courses sort in alphabetical order is a sensible default that doesn't require much effort to implement.