Twenty Fifteen: use better, universal sans-serif fonts for RTL, similar to Twenty Ten which also had serif fonts throughout.

Merges [30961] to the 4.1 branch.

Props iamtakashi
Fixes #30752.

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git-svn-id: http://core.svn.wordpress.org/branches/4.1@30950 1a063a9b-81f0-0310-95a4-ce76da25c4cd