Refactor Drag and Drop logic

[mirmirmirr]

Throughout the codebase, the drag and drop and course moving logic is scattered and duplicated. It would be nice to centralize this for future-proofing.

For example, the following things might be addressed:

• There is a lot of prop drilling with the following elements from usePlannerCourse:
  • setPlannerCourses, setToolboxCourses, semesterIndex, courseIndex, course, id, name, count
  • this goes from the top-level Planner.tsx down to each individual PlannerCourse component
• The useDragAndDrop.ts context includes a lot of repeating logic and continuously re-renders setPlannerCourse. It might be better to convert this into a reducer.