Fix modal window stickiness

Previously the modal window appeared at a specific point in the page and
you could scroll away from it. This makes it always in the center of the
page.

src/audio_feeder/data/css/main.css

@@ -276,7 +276,7 @@ div.topnav-icons {
 
 .modal-window {
     display: inline-block;
-    position: absolute;
+    position: fixed;
     top: 50%;
     left: 50%;
     transform: translate(-50%, -50%);