Accessibility: Administration: Improve the sortable postboxes areas on the...

Accessibility: Administration: Improve the sortable postboxes areas on the Dashboard and Classic Editor pages.

- makes the postboxes areas in the Dashboard visible also on large screens 
- uses a more meaningful text when all postboxes areas are empty instead of "Drag boxes here"
- restores the ability to drag boxes to the "advanced" area in the Classic Editor page
- makes the postboxes areas in the Classic Editor page visible while dragging so that users have a clue what the available areas are
- improves the color contrast of the postboxes areas while dragging
- uses `wp.i18n` for translatable strings in `wp-admin/js/postbox.js`

Props xkon, karmatosed, audrasjb, ocean90, joedolson, afercia, azaozz.
See #20491.
Fixes #49288, #47541.

Built from https://develop.svn.wordpress.org/trunk@48340


git-svn-id: https://core.svn.wordpress.org/trunk@48109 1a063a9b-81f0-0310-95a4-ce76da25c4cd