[judge] Undo vote

from November 21, 2020 test-run for HackTJ 7.5: undoing vote is a very important UX factor

we need to:
- show a toast notification saying `Voted for {{ better overall project }}. Undo` using django messages
- undo changes to statistical distributions

linking https://github.com/anishathalye/gavel/issues/22 ("Another way we could do it is by replaying all the decisions with the cancelled ones removed (this would be an O(n) operation, and for hackathon scale, probably fine)")