Change spinner to progress circle when downloading an episode.

[mikece]

On the "Episodes" screen, when a user clicks on the icon for downloading an episode a spinner is displayed. This is confusing because it's not clear if the app is trying to reach the server to download a file, downloading the file, waiting for system resources, etc. If possible, please replace the spinner with a progress circle -- something like this: https://www.npmjs.com/package/react-native-progress-circle or https://www.npmjs.com/package/react-native-progress -- so one could see what the download progress is right on the episodes tab.

[mitchdowney]

This would be neat. I'm not sure when it makes it to the top of our priorities, but it'd be a nice UX improvement.