comments: display comment previews while submitting

Instead of just saying 'Submitting' and not showing any progress to
the user until the comment has been accepted by the server, show a
preview of the comment above the comment box in a way which is makes
it obvious to the user the comment is being submitted. Apart from
that, also clear the comment box so that a repeated clicking the
submit button doesn't result in a duplicate comment.

The preview doesn't highlight URLs or support @mentions or *bold*,
which is a good enough approximation in this case. When/if we (re-)add
the rST/Markdown support, we will need a client-side parser for the
syntax we choose.

When the submission fails, display a message and offer the user to
retry or cancel the submission.