Twenty Fifteen: Add Foursquare icon support to the social link menu. 

props iamtakashi.
fixes #30100.
Built from https://develop.svn.wordpress.org/trunk@30023


git-svn-id: http://core.svn.wordpress.org/trunk@30023 1a063a9b-81f0-0310-95a4-ce76da25c4cd