As mentioned in commits 5311daadf and bb958daf0, using links combined
with JavaScript to generate POST requests to the server has a few
issues.
We're also improving the keyboard access. Previously, the links were
focusable and clickable with the keyboard. Now we're disabling the
buttons when voting isn't allowed.
Since these elements can no longer be focused, we're adding an element
with `tabindex="0"` so the "participation not allowed" message is shown,
like we do in most places.
Note we're slightly changing one test because now when hovering over the
button on Chrome, the "participation not allowed" text isn't shown; it's
only shown when hovering on the parts of the `div.ballot` element
outside the button. Since we're already rewriting the behavior of the
"participation not allowed" text in a different pull request, we aren't
going to fix this behavior.
7a25c61ae6