R1:26081594e2f3

Fix two very, very minor correctness issues in Slowvote

Summary:
See <https://hackerone.com/reports/492525> and <https://hackerone.com/reports/489531>. I previously awarded a bounty for <https://hackerone.com/reports/434116> so Slowvote is getting "researched" a lot.

- Prevent users from undoing their vote by submitting the form with nothing selected.
- Prevent users from racing between the `delete()` and `save()` to vote for multiple options in a plurality poll.

Test Plan:
- Clicked the vote…