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

props iamtakashi.
fixes #30100.

git-svn-id: https://develop.svn.wordpress.org/trunk@30023 602fd350-edb4-49c9-b593-d223f7449a82
Fix JSHint errors introduced in [30014].

props adamsilverstein.
see #29572.

git-svn-id: https://develop.svn.wordpress.org/trunk@30024 602fd350-edb4-49c9-b593-d223f7449a82