Add postname to Settings > Permalinks and remove the help text talking about...

Add postname to Settings > Permalinks and remove the help text talking about permalink performance. Make the slugs (and /archives/ rewrite base) translatable. see #18541.

git-svn-id: https://core.svn.wordpress.org/trunk@18547 1a063a9b-81f0-0310-95a4-ce76da25c4cd