fix: Color Changing Bug w/ Duplicate Courses

[KevinWu098]

Summary

screen-recorder-sat-sep-30-2023-15-39-24.1.webm

As shown above (and in #716), changing colors of a course event fails if the same course event exists in a schedule that is lower in index than the one currently being changed.

The reason for this is that getExistingCourse (shown below) only returned the reference of a singular course that matched the given params. This meant that courses (which matched the params) in other schedules would not have their colors changed.

    /**
     * @return Reference of a course that matches the params
     */
    getExistingCourse(sectionCode: string, term: string) {
        for (const course of this.getAllCourses()) {
            if (course.section.sectionCode === sectionCode && term === course.term) {
                return course;   // <----- the problem
            }
        }
        return undefined;
    }

TL;DR Color Picker and Courses will now correctly update their colors (without affecting other schedules and courses)

Test Plan

1. Create at least two schedules which contain the same course
2. Attempt to change the color on each. Success will be defined by steps 3-6
3. The color of the course in the Calendar should update
4. The color of the course in the Search pane should update
5. The color of the course in the Added pane should update
6. The updated colors should save and be loaded in correctly

Issues

Closes #716

[EricPedley]

Results look good, just a few comments about the implementation

[EricPedley]

🚢🚢🚢