For the longest time, we've disabled the buttons to vote via web when
people had already voted in a booth. However, we were still allowing
HTTP requests to the actions to vote via web.
So we're adding a condition to prevent it.
The reason why we're changing the controller instead of the abilities
model (which is what we usually do) is that there might be side-effects
to the change. For instance, in the `Polls::PollComponent` class,
there's an `elsif cannot?(:answer, poll)` condition which would have a
different behavior if we changed the abilities model.
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