[6.x] Add z-index to navbar in CP

[wiktorm12]

Fix grid header overlaps navigation

Before:

After:

Fixes #14608

[jackmcdade]

Thanks for giving this a go! We try not to use arbitrary z-index values but rather have vars to map out the various proper z-indexes in resources/css/elements/base.css

--z-index-below: -1;
--z-index-above: 1;
--z-index-portal: 2;
--z-index-draggable: 3;
--z-index-modal: 4;
--z-index-max: 5;

You up for taking a stab w/ those or should I take it from here?