Show a warning when adding a class to a schedule with classes in a different semester

[Samathingamajig]

Probably should have buttons like "Cancel", "Add anyways", and "Add to new schedule" ('default' button)

For add to schedule, it should create a new schedule (Name it something like "Fall 2025" if the semester code is 20259), make it be the active schedule. If doing this would make more than 10 schedules, go ahead and allow it (don't show the 10 schedules warning, that would be a bad UX) (you might need to add an argument to the create schedule function, not sure).

Related: #451