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/install.php

@@ -141,7 +141,7 @@ function display_setup_form( $error = null ) {
 				<div class="">
 					<?php $initial_password = isset( $_POST['admin_password'] ) ? stripslashes( $_POST['admin_password'] ) : wp_generate_password( 18 ); ?>
 					<input type="password" name="admin_password" id="pass1" class="regular-text" autocomplete="off" data-reveal="1" data-pw="<?php echo esc_attr( $initial_password ); ?>" aria-describedby="pass-strength-result" />
-					<button type="button" class="button button-secondary wp-hide-pw hide-if-no-js" data-start-masked="<?php echo (int) isset( $_POST['admin_password'] ); ?>" data-toggle="0" aria-label="<?php esc_attr_e( 'Hide password' ); ?>">
+					<button type="button" class="button wp-hide-pw hide-if-no-js" data-start-masked="<?php echo (int) isset( $_POST['admin_password'] ); ?>" data-toggle="0" aria-label="<?php esc_attr_e( 'Hide password' ); ?>">
 						<span class="dashicons dashicons-hidden"></span>
 						<span class="text"><?php _e( 'Hide' ); ?></span>
 					</button>