Use theme-color to color address bar in browsers

This makes the address bar on mobile browsers match the background. Note
that it requires a valid certificate and a non-dark mode set on the
device. Not supported on desktop browsers.

vnc.html

@@ -16,6 +16,7 @@
     <title>noVNC</title>
 
     <link rel="icon" type="image/x-icon" href="app/images/icons/novnc.ico">
+    <meta name="theme-color" content="#313131">
 
     <!-- Apple iOS Safari settings -->
     <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">