Networks and Sites: Reduce `max-width` for user and site search inputs on...

Networks and Sites: Reduce `max-width` for user and site search inputs on Dashboard screen in Network Admin.

This ensures that the inputs don't push the buttons to the next line.

Props williampatton, bordoni.
Fixes #49043.
Built from https://develop.svn.wordpress.org/trunk@47151


git-svn-id: https://core.svn.wordpress.org/trunk@46951 1a063a9b-81f0-0310-95a4-ce76da25c4cd