Add sitesub, but hide by default

Bug: T193215
Change-Id: Ia09ae2b763107d0bdc3c379525405e8edf17edb7

includes/TimelessTemplate.php

@@ -50,6 +50,7 @@ class TimelessTemplate extends BaseTemplate {
 								],
 								$this->get( 'title' )
 							) .
+							Html::rawElement( 'div', [ 'id' => 'siteSub' ], $this->getMsg( 'tagline' )->parse() ) .
 							Html::rawElement( 'div', [ 'id' => 'mw-page-header-links' ],
 								$this->getPortlet(
 									'namespaces',

resources/screen-common.less

@@ -391,6 +391,13 @@ pre,
 	}
 }
 
+#siteSub {
+	margin: -0.5em 0 0.5em 0;
+	font-style: italic;
+	font-family: @fonts-secondary;
+	display: none;
+}
+
 hr {
 	border-width: 1px;
 	border-color: @grey;