Restore video loading circle

## Issue
5554: Video: loading circle sometimes does not appear until 2nd click

## What's happening
videojs behavior:
(a) A `src` change makes the Play button re-appear.
(b) An `onPlay` (or `play()`) makes the button go away.

Due to the `m3u8` header async fetch (i.e. return is potentially delayed), the initial `onPlay` (which cleared the button) that happened after user clicked Play gets negated by a potentially-delayed `src` change.

# Changes
- Manually hide the play button that is induced by the change in `src`. In the fetch-delay scenario mentioned above, the player continues to be in a 'playing' state anyway.
- But don't hide the button if paused externally (e.g. browser-level)
Restore video loading circle

ui/component/viewers/videoViewer/internal/videojs.jsx

@@ -453,6 +453,11 @@ export default React.memo<Props>(function VideoJs(props: Props) {
         src: source,
         type: type,
       });
+
+      // PR #5570: Temp workaround to avoid double Play button until the next re-architecture.
+      if (!player.paused()) {
+        player.bigPlayButton.hide();
+      }
     });
   }, [source, reload]);