Accessibility: Plugins: Accessibility and CSS improvements for the Plugins pages.

- improves checkboxes alignment on the "Plugins" page table in the responsive view 
- improves spacing between form controls on the "Add Plugins" page in the responsive view 
- the layout of the "filter bar" on the "Add Plugins" page is now based on CSS Flexbox 
- removes italic type from a paragraph in the "Favorites" page

Props passoniate, garethgillman, maxpertici, audrasjb, sabernhardt, afercia.
See #47327.
Fixes #49231.


git-svn-id: https://develop.svn.wordpress.org/trunk@48281 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/plugin-install.php

@@ -158,7 +158,6 @@ if ( 'upload' !== $tab ) {
 	</div>
 	<?php
 	$wp_list_table->views();
-	echo '<br class="clear" />';
 }
 
 /**