Filter bar: Better field display on smaller screens.

props TacoVerdo, jesin.
fixes #29556 and #29495.

Built from https://develop.svn.wordpress.org/trunk@30740


git-svn-id: http://core.svn.wordpress.org/trunk@30730 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Fifteen: updating customizer label text for more clarity.

Props iamtakashi, fixes #30545.


Built from https://develop.svn.wordpress.org/trunk@30741


git-svn-id: http://core.svn.wordpress.org/trunk@30731 1a063a9b-81f0-0310-95a4-ce76da25c4cd