Bundled Themes: Twenty Seventeen Navbar z-index issue.

Fixes navbar z-index issue by upping the z-index value to 1000 from 7.

Props halgatewood, justnorris, poena, sinatrateam.
Fixes #39384.
Built from https://develop.svn.wordpress.org/trunk@48061


git-svn-id: https://core.svn.wordpress.org/trunk@47828 1a063a9b-81f0-0310-95a4-ce76da25c4cd