Accessibility: Improve accessibility of all the media views form controls.

- changes the media views form controls to have explicitly associated labels with for/id attributes
- adds a few missing labels / aria-labels
- improves a few existing labels / aria-labels
- improves semantics in a few places, by adding visually hidden headings, fieldset + legend elements, aria-describedby attributes
- improves the image custom size input fields and their labelling
- adds `role="status"` to the "saved" indicator so that status messages are announced to assistive technologies
- swaps the columns source order in the image details template, to make visual and DOM order match
- swaps the "Replace" and "Back" buttons source order in the Replace Image view, to make visual and DOM order match
- gallery settings: move checkbox label to the right: checkboxes are supposed to have labels on the right
- merge similar strings, unified to "Drop files to upload" (removed "Drop files here", and "Drop files anywhere to upload")
- makes the "upload-ui" consistent across the media views
- hides the IE 11 "X" `::-ms-clear` button in the Insert from URL field, as it conflicts with the uploading spinner
- adds comments to all the media templates to clarify their usage
- slightly increases vertical spacing between form fields in the media sidebar
- removes some CSS selectors introduced as backwards compatibility for WordPress pre-4.4
- removes some CSS still targeting Internet Explorer 7 and 8

Fixes #47141.
Fixes #47122.

Built from https://develop.svn.wordpress.org/trunk@45499


git-svn-id: http://core.svn.wordpress.org/trunk@45310 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/css/buttons-rtl.css

@@ -309,7 +309,6 @@ TABLE OF CONTENTS:
 	display: inline-block;
 	border-radius: 0;
 	margin-left: -1px;
-	z-index: 10;
 }
 
 .wp-core-ui .button-group > .button-primary {