Accessibility: Theme Installer, make the "Upload Theme" button... a button.

UI controls that "do something" on a page shouldn't be links. This link behaves
like a toggle to expand the uploader panel and should be a `button` element with
an aria-expanded attribute. Also:

- improves consistency with the Plugin uploader
- keeps the themes list visible when the uploader is open
- displays a notice when JavaScript is off
- adds some `hide-if-no-js` CSS classes
- removes the `themes.router.navigate()` "upload" route: seems unnecessary and breaks history

Fixes #35457.
Built from https://develop.svn.wordpress.org/trunk@37742


git-svn-id: http://core.svn.wordpress.org/trunk@37707 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/css/themes.css

@@ -1039,7 +1039,7 @@ body.folded .theme-browser ~ .theme-overlay .theme-wrap {
 	box-sizing: border-box;
 	display: none;
 	margin: 0;
-	padding: 0;
+	padding: 50px 0;
 	width: 100%;
 	overflow: hidden;
 	position: relative;
@@ -1066,13 +1066,9 @@ body.folded .theme-browser ~ .theme-overlay .theme-wrap {
 	font-size: 18px;
 	font-style: normal;
 	margin: 0;
-	padding: 40px 0 0;
+	padding: 0;
 	text-align: center;
 }
-body.show-upload-theme .upload-theme + .wp-filter,
-body.show-upload-theme .upload-theme + .wp-filter + .theme-browser {
-	display: none;
-}
 
 p.no-themes {
 	clear: both;
@@ -1085,14 +1081,10 @@ p.no-themes {
 	display: none;
 }
 
-body.no-results p.no-themes {
+.no-results p.no-themes {
 	display: block;
 }
 
-body.show-upload-theme p.no-themes {
-	display: none !important;
-}
-
 .theme-install-php .add-new-theme {
 	display: none !important;
 }