Simplify comment labeling in image.php

The image template is the only one to label "comments and trackbacks are open" "comments are closed but trackbacks are open" etc. ad nauseam. Few people are using those options any more, and it's a lot of clutter on a template that (if anything) should be simpler and cleaner than all the rest - we should delete all that and match the comment presentation to the way it's done in page.php and single.php
Merge pull request #306 from sabreuse/patch-1

_s: Simplify comment labeling.
_s: Make footer markup optional with edit post link.
_s: Be more specific about clearing elements.

(Introduced in b1d3b53)

While class attribute selectors are very powerful and might help in
keeping stylesheets concise and easier to maintain, they don't work very
well with a project like WordPress. Not only can third-party scripts
insert and rely on specific class names that get picked up by attribute
selectors for common class names. But more importantly WordPress passes
category and tag slugs on to post classes, which can result in
unpredictible results.

Fixes #284, fixes #309.