R1:d38e768ed877

Prevent users from voting for invalid Slowvote options

Summary:
Depends on D19773. See <https://hackerone.com/reports/434116>. You can currently vote for invalid options by submitting, e.g., `vote[]=12345`.

By doing this, you can see the responses, which is sort of theoretically a security problem? This is definitely a bug, regardless.

Instead, only allow users to vote for options which are actually part of the poll.

Test Plan:
- Tried to vote for invalid options by editing the form to `vote[]=12345`…