Twenty Twelve and Thirteen: Use proper way to get customizer settings values.

Props obenland, kdoran. Fixes #29577.
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Media a11y: Don't set role=checkbox for the attachment details view.

Some unnecessary attributes were inherited from the attachment view, which created difficulties with screen readers.

fixes #30390.

Built from https://develop.svn.wordpress.org/trunk@30483


git-svn-id: http://core.svn.wordpress.org/trunk@30471 1a063a9b-81f0-0310-95a4-ce76da25c4cd
4.1-beta2-30484

Built from https://develop.svn.wordpress.org/trunk@30484


git-svn-id: http://core.svn.wordpress.org/trunk@30473 1a063a9b-81f0-0310-95a4-ce76da25c4cd