Prevent default event handling when clicking on the video position slider

This prevents accidental text selection and avoids opening the toot containing
the video in the dynamic column when clicking on the position slider.

app/javascript/flavours/glitch/features/video/index.js

@@ -160,6 +160,9 @@ export default class Video extends React.PureComponent {
     this.setState({ dragging: true });
     this.video.pause();
     this.handleMouseMove(e);
+
+    e.preventDefault();
+    e.stopPropagation();
   }
 
   handleMouseUp = () => {