Add some basic multiple voting checks.

[zorn]

We want to discourage people from voting twice, however we have a limited appetite for how strongly we want to enforce this.

As a simple first pass we'd like to do two things:

1. Store in the session that the user has voted for ballot X and if so tell them they already recorded.
2. Store the IP with the vote, if attempting to render the vote capture page for an IP that has already voted, don't let them.

We should be vague in the messaging so as to not encourage people to recycle their IP.

These are notable frail security measures but help protect against lazy people who want to vote more than once.