Media: Fix bottom spacing on various Media Modal elements for non-webkit browsers.

Implementation of bottom padding in overflow content differs across browsers. See https://github.com/w3c/csswg-drafts/issues/129. To make bottom spacing consistent across browsers there's the need for an alternate CSS method.
- uses a CSS `after` pseudo element or simply a bottom margin to reserve some bottom spacing
- removes a couple leftovers from [40428]
- fixes an annoying visual glitch where the media modal content is visible behind the bottom toolbar border

Props sabernhardt, afercia.
See #40152.
Fixes #48378.

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git-svn-id: https://core.svn.wordpress.org/trunk@47066 1a063a9b-81f0-0310-95a4-ce76da25c4cd