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

Merges [26920] to the 3.8 branch.

props iammattthomas.
fixes #26537.


git-svn-id: https://develop.svn.wordpress.org/branches/3.8@26949 602fd350-edb4-49c9-b593-d223f7449a82