Bundled Themes: Twenty Nineteen long submenu item titles do not wrap.

Fixes the issue of long submenu item titles that do not wrap by adding a `max-width: 20rem;` and removing `white-space: nowrap;`.

Props kjellr, mukesh27, mohsinrasool, samful.
Fixes #45866.
Built from https://develop.svn.wordpress.org/trunk@48032


git-svn-id: https://core.svn.wordpress.org/trunk@47799 1a063a9b-81f0-0310-95a4-ce76da25c4cd