Twenty Eleven: miscellaneous bug fixes and improvements

* Only style images uploaded to the WordPress media library with border styles (and not placeholder images for content added by plugins)
* Prevent content added by plugins and long custom excerpts from overflowing the featured post slider
* Make sure small thumbnails will never appear in the Custom Header area
* Better indicate the current featured post with custom link color and cursor
* Add a class for text-only featured posts
* Style improvements for small screens


git-svn-id: https://core.svn.wordpress.org/trunk@18181 1a063a9b-81f0-0310-95a4-ce76da25c4cd