With the old interface, there wasn't a clear way to send a blank ballot.
But now that we've got a form, there's an easy way: clicking on "Vote"
while leaving the form blank.
Some of the code was in its own component, while some of the code
remained in the polls/show view.
Note that we're re-structuring the code a little bit, so it's clear that
the "already voted" messages are only shown when users can vote. Also
note that now the `can?` condition involves the existence of a
`current_user` and that the poll is not expired, so we can simplify the
`voted_in_web` condition.
This way it'll be easier to refactor it.
Note there was a system test which tested both the callout and the form
when unverified users visit a poll. We've split this system test in two
component tests.
