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.

Built from https://develop.svn.wordpress.org/trunk@38672


git-svn-id: http://core.svn.wordpress.org/trunk@38615 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/network/themes.php

@@ -158,9 +158,9 @@ if ( $action ) {
 						wp_nonce_field( 'bulk-themes' );
 
 						if ( 1 == $themes_to_delete ) {
-							submit_button( __( 'Yes, delete this theme' ), 'button', 'submit', false );
+							submit_button( __( 'Yes, delete this theme' ), '', 'submit', false );
 						} else {
-							submit_button( __( 'Yes, delete these themes' ), 'button', 'submit', false );
+							submit_button( __( 'Yes, delete these themes' ), '', 'submit', false );
 						}
 					?>
 				</form>
@@ -168,7 +168,7 @@ if ( $action ) {
 				$referer = wp_get_referer();
 				?>
 				<form method="post" action="<?php echo $referer ? esc_url( $referer ) : ''; ?>" style="display:inline;">
-					<?php submit_button( __( 'No, return me to the theme list' ), 'button', 'submit', false ); ?>
+					<?php submit_button( __( 'No, return me to the theme list' ), '', 'submit', false ); ?>
 				</form>
 			</div>
 				<?php