Twenty Fifteen: ensure RTL editor font style is consistent with front-end font style.

Props iamtakashi and alex-ye, see #30746.
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Update version numbers in trunk.

Built from https://develop.svn.wordpress.org/trunk@30960


git-svn-id: http://core.svn.wordpress.org/trunk@30948 1a063a9b-81f0-0310-95a4-ce76da25c4cd