fix: default colors deplete too rapidly

[matt-franklin225]

Summary

Adjusted color selection to work by linear assignment from the 8th course onward. This is done by determining the color of the most recent class added and choosing the color directly after it in the defaultColors list (looping back around to the first color when needed).

Test Plan

• Ensure that the assignment works properly when classes are deleted out of order.
• Ensure assignment works when users select custom colors for courses (this in particular poses an issue).

Issues

Closes #647

Future Followup

There can still be some imbalance in colors if many classes are added and some deleted--if keeping all the colors even is a priority we could auto-update the colors of previously added classes, but that may be unnecessary or overkill. Can also add to the original 7-color palette if desired.