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/includes/class-wp-plugins-list-table.php

@@ -367,7 +367,7 @@ class WP_Plugins_List_Table extends WP_List_Table {
 		<p class="search-box">
 			<label class="screen-reader-text" for="<?php echo esc_attr( $input_id ); ?>"><?php echo $text; ?>:</label>
 			<input type="search" id="<?php echo esc_attr( $input_id ); ?>" class="wp-filter-search" name="s" value="<?php _admin_search_query(); ?>" placeholder="<?php esc_attr_e( 'Search installed plugins...' ); ?>"/>
-			<?php submit_button( $text, 'button hide-if-js', '', false, array( 'id' => 'search-submit' ) ); ?>
+			<?php submit_button( $text, 'hide-if-js', '', false, array( 'id' => 'search-submit' ) ); ?>
 		</p>
 		<?php
 	}
@@ -496,7 +496,7 @@ class WP_Plugins_List_Table extends WP_List_Table {
 		echo '<div class="alignleft actions">';
 
 		if ( 'recently_activated' == $status ) {
-			submit_button( __( 'Clear List' ), 'button', 'clear-recent-list', false );
+			submit_button( __( 'Clear List' ), '', 'clear-recent-list', false );
 		} elseif ( 'top' === $which && 'mustuse' === $status ) {
 			/* translators: %s: mu-plugins directory name */
 			echo '<p>' . sprintf( __( 'Files in the %s directory are executed automatically.' ),