Code review comment for lp:~bjornt/launchpad/add-comment-submit-button

Replace the 'add comment' link with a submit button to make it more
obvious how to add a comment.

The labels are also renamed, so that the heading is "Add comment" and
the button reads "Post comment." The latter causes test failures of
course, and I intend to fix them, but I don't think it will affect the
review.

As per discussion with Martin, I also made it so that the submit button
is disabled, if there is no text in the comment box.

--
Björn Tillenius | https://launchpad.net/~bjornt