Center Buttons

Since Version 18.0.0 the Registration button of the [registration app](https://github.com/pellaeon/registration) is not centered anymore.

core/css/guest.css

@@ -193,7 +193,7 @@ form #datadirField legend {
 		margin-top: 0;
 	}
 	#alternative-logins {
-		margin: 30px 15px 10px;
+		margin: auto;
 	}
 }