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/customize/class-wp-customize-new-menu-section.php

@@ -36,7 +36,7 @@ class WP_Customize_New_Menu_Section extends WP_Customize_Section {
 	protected function render() {
 		?>
 		<li id="accordion-section-<?php echo esc_attr( $this->id ); ?>" class="accordion-section-new-menu">
-			<button type="button" class="button-secondary add-new-menu-item add-menu-toggle" aria-expanded="false">
+			<button type="button" class="button add-new-menu-item add-menu-toggle" aria-expanded="false">
 				<?php echo esc_html( $this->title ); ?>
 			</button>
 			<ul class="new-menu-section-content"></ul>