Twenty Fifteen: add a fallback icon for social links where we don't have an icon

Props iamtakashi, fixes #30148


Built from https://develop.svn.wordpress.org/trunk@30212


git-svn-id: http://core.svn.wordpress.org/trunk@30212 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: nicer styles for playlists.

Props iamtakashi, fixes #30163


Built from https://develop.svn.wordpress.org/trunk@30213


git-svn-id: http://core.svn.wordpress.org/trunk@30213 1a063a9b-81f0-0310-95a4-ce76da25c4cd