Administration: Improve the usage of the button CSS classes.

Introduces some consistency in the usage of the button CSS classes, fixes the
focus style for accessibility and responsiveness of the buttons.

- Adds the `button` class to all primary buttons make them responsive
- Removes all `secondary-button` classes and replaces it with button when needed. `button-secondary` shouldn't be used and exists just for backward compatibility reasons
- Replaces classes inside `submit_button()` with a shorthand for some buttons, and use an empty string for the default `button` class. Passing `button` is unnecessary
- Adjusts `get_submit_button()` to remove empty items

Props iseulde, dimchik, chris_d2d, mhowell, afercia.
Fixes #27314, #37138, #37448.


git-svn-id: https://develop.svn.wordpress.org/trunk@38672 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-customize-widgets.php

@@ -712,7 +712,7 @@ final class WP_Customize_Widgets {
 					<% }); %>
 				</ul>
 				<div class="move-widget-actions">
-					<button class="move-widget-btn button-secondary" type="button">{btn}</button>
+					<button class="move-widget-btn button" type="button">{btn}</button>
 				</div>
 			</div>'
 		);