Prevent referer from being sent

Nice hardening for enhanced privacy. Especially useful when using embedded viewers such as files_pdfviewer.

core/templates/layout.base.php

@@ -8,6 +8,7 @@
 		<?php p($theme->getTitle()); ?>
 		</title>
 		<meta http-equiv="X-UA-Compatible" content="IE=edge">
+		<meta name="referrer" content="never">
 		<meta name="viewport" content="width=device-width, minimum-scale=1.0, maximum-scale=1.0">
 		<meta name="theme-color" content="<?php p($theme->getMailHeaderColor()); ?>">
 		<link rel="shortcut icon" type="image/png" href="<?php print_unescaped(image_path('', 'favicon.png')); ?>">

core/templates/layout.guest.php

@@ -8,6 +8,7 @@
 		<?php p($theme->getTitle()); ?>
 		</title>
 		<meta http-equiv="X-UA-Compatible" content="IE=edge">
+		<meta name="referrer" content="never">
 		<meta name="viewport" content="width=device-width, minimum-scale=1.0, maximum-scale=1.0">
 		<meta name="apple-itunes-app" content="app-id=<?php p($theme->getiTunesAppId()); ?>">
 		<meta name="theme-color" content="<?php p($theme->getMailHeaderColor()); ?>">

core/templates/layout.user.php

@@ -15,6 +15,7 @@
 			?>
 		</title>
 		<meta http-equiv="X-UA-Compatible" content="IE=edge">
+		<meta name="referrer" content="never">
 		<meta name="viewport" content="width=device-width, initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0">
 		<meta name="apple-itunes-app" content="app-id=<?php p($theme->getiTunesAppId()); ?>">
 		<meta name="apple-mobile-web-app-capable" content="yes">