Twenty Fifteen: remove license details for the image shown in the theme  screenshot file as it is Creative Commons Zero, which doesn't require attribution.

Props iamtakashi, fixes #30156.
Built from https://develop.svn.wordpress.org/trunk@30867


git-svn-id: http://core.svn.wordpress.org/trunk@30857 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Twelve: fix widget stacking problem in older IE browsers for Front Page template.

Props TomasM, hiwhatsup. Fixes #28938.
Built from https://develop.svn.wordpress.org/trunk@30868


git-svn-id: http://core.svn.wordpress.org/trunk@30858 1a063a9b-81f0-0310-95a4-ce76da25c4cd