Fixed placement/position of image embed buttons.

The "Embed" button currently appears below an attached file.  For consistency, it should be placed/positioned in the exact same way as the patch review/simplytest.me buttons for patch files.

While being there, also fixing the selector + .once() filtering in the same way as in #98, since the behavior is needlessly iterating over links that have been injected by Dreditor. — I'm fairly sure that this is actually the proper fix for the recently committed adjustment that changed the link into a span, but not changing/reverting that here, since a span works, too.