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.


git-svn-id: https://develop.svn.wordpress.org/branches/4.1@30962 602fd350-edb4-49c9-b593-d223f7449a82