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-admin/customize.php

@@ -168,7 +168,7 @@ do_action( 'customize_controls_print_scripts' );
 				<?php endforeach; ?>
 			</div>
 			<?php endif; ?>
-			<button type="button" class="collapse-sidebar button-secondary" aria-expanded="true" aria-label="<?php esc_attr_e( 'Collapse Sidebar' ); ?>">
+			<button type="button" class="collapse-sidebar button" aria-expanded="true" aria-label="<?php esc_attr_e( 'Collapse Sidebar' ); ?>">
 				<span class="collapse-sidebar-arrow"></span>
 				<span class="collapse-sidebar-label"><?php _e( 'Collapse' ); ?></span>
 			</button>