This way it'll be easier for people using screen readers to know which
element the checkbox is related to.
Note that we're using the `aria-label` attribute because it makes
testing with Capybara easier than using the `aria-labelledby` attribute.
The only exception are the comments, since comments don't have a title
and there isn't a proper label for them. In this case, we're using the
title of the associated commentable as the label; we might change it in
the future since there might be many comments for the same commentable.
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