Reject additional votes

Depends on #208

Finally starting to clamp down on the API by rejecting new votes once the user has already voted unless/until they remove the blocking vote.

This exposes a TON of weaknesses in our code from our accumulated technical debt we built up for demo day.

Things still needing to be done:

~~1) Create a /changeVote or /updateVote API endpoint to prevent a race condition with multiple-choice-single-response~~ (Issue #216)
~~2) Create an endpoint to get the user's current votes~~ (Issue #217)
~~3) Add support for all of that to the client.~~ (Issue #218)

This pull is set to merge into the sessions branch of upstream, since this will require a team effort to fix and this is not stable in its current condition.