Adjust widths of toolbar items at mobile sizes for better fitting on narrow...

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