Twenty Thirteen: improvements to Image post format handling to use a custom image size instead of filtering a `$content_width` variable.

Includes CSS cleanup and markup improvements to both video and image templates, moving the media HTML piece out of the `header` element.

Props obenland, closes #23620.

git-svn-id: http://core.svn.wordpress.org/trunk@23839 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Move the "look for the first media attachment" code below the "look
in the post content for media" code, so that it is the method of last
resort. This way, if you attach AND embed media, you won't get duplication.

fixes #23879

git-svn-id: http://core.svn.wordpress.org/trunk@23840 1a063a9b-81f0-0310-95a4-ce76da25c4cd