[judge] Undo vote

[sumanthratna]

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 }}. <button class="style-as-link with tailwind">Undo</button> using django messages
• undo changes to statistical distributions

linking anishathalye/gavel#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)")