Twenty Eleven updates and fixes, see #17198

 * Add explanatory comments in all templates, following Twenty Ten's example
 * Stick with twentyeleven_ for prefixing filter names
 * Use post format content templates in search results
 * First pass at adding a template for adding a sidebar to pages
 * Add a featured post header to standard sticky posts
 * Style fixes: print media, comment styles for pages with sidebars, color of meta links on image post format

git-svn-id: https://core.svn.wordpress.org/trunk@17802 1a063a9b-81f0-0310-95a4-ce76da25c4cd