Use is_rtl() for html direction when possible. In sites.php,...

Use is_rtl() for html direction when possible. In sites.php, language_attributes() will always exist (MU vestige). When the DB is dead, language_attributes() is worse than a simple is_rtl() check. is_rtl() exists here due to wp_load_translations_early(). see #18180.

git-svn-id: https://core.svn.wordpress.org/trunk@19862 1a063a9b-81f0-0310-95a4-ce76da25c4cd