Add .mw-footer CSS class & replace #footer selector with it

`#footer` kept for backwards compatibility with user styles and gadgets.

Bug: T248137
Change-Id: Ied8e79c5895f8fe383b1ca743a31890549cc8d86

includes/MonoBookTemplate.php

@@ -589,6 +589,7 @@ class MonoBookTemplate extends BaseTemplate {
 
 		$html .= Html::openElement( 'div', [
 			'id' => 'footer',
+			'class' => 'mw-footer',
 			'role' => 'contentinfo',
 			'lang' => $this->get( 'userlang' ),
 			'dir' => $this->get( 'dir' )

resources/screen-common.less

@@ -225,7 +225,7 @@ li#pt-anonuserpage {
 /*
 ** footer
 */
-#footer {
+.mw-footer {
 	background-color: @content-background;
 	border-top: 1px solid @orange-border;
 	border-bottom: 1px solid @orange-border;

resources/screen-mobile.less

@@ -241,7 +241,7 @@
 /*
 ** footer
 */
-#footer li {
+.mw-footer li {
 	margin-left: 0;
 }