Add icon to admin page sidebar

* follow up to nextcloud/server#3151

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

Author: MorrisJobke

css/style.css

@@ -18,4 +18,9 @@
 	vertical-align: sub;
 	margin-left: -2px;
 	margin-right: 3px;
+}
+
+/** icons for personal page settings **/
+.nav-icon-u2f-second-factor-auth {
+	background-image: url('../img/app-dark.svg?v=1');
 }