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 button with nothing selected in plurality and approval polls, got an error now.
• Added a sleep(5) between delete() and save(). Submitted different plurality votes in different windows. Before: votes raced, invalid end state. After: votes waited on the lock, arrived in a valid end state.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20125