feat: improvements to save state

[KevinWu098]

Summary

1. Prevents "undo" if it would return the user to an empty state. This prevents autosave from wiping the user's schedule since we can never return to an empty state
2. Clears "states" when loading in a new username. This prevents undoing to another user's schedule and having that autosaved.

Test Plan

1. Load a schedule. Try and undo (ctrl + z). Nothing should happen.
2. Undoing should still work, but not on the very first action you take
3. Load a schedule. Load a different schedule. Undo should do nothing.

Issues

Closes #1133

[MinhxNguyen7]

LGTM!