Adjust widths of toolbar items at mobile sizes for better fitting on narrow screens. Fixes toolbar wrapping when using multisite.

props iammattthomas.
see #26537 for trunk.

Built from https://develop.svn.wordpress.org/trunk@26920


git-svn-id: http://core.svn.wordpress.org/trunk@26801 1a063a9b-81f0-0310-95a4-ce76da25c4cd