Wrap download logic in streamlit fragments

[michalismeng]

What's changing

Wrap the download buttons for the podcast script and audio files into streamlit fragments. This way we avoid refreshing the application when clicking those buttons. This also addresses comment #42 (review), about losing the text and audio snippets in the UI (because they are not persisted), since we no longer refresh the app.

Closes #71

How to test it

Steps to test the changes:

1. Run the demo to generate a podcast.
2. Click on the "Save Podcast to audio file".
3. Verify the page won't refresh. In addition, verify the text and audio snippets of the podcast won't disappear.
4. Do the same for the "Save Podcast script to text file".

Additional notes for reviewers

I already...

• Tested the changes in a working environment to ensure they work as expected
• Added some tests for any new functionality
• Updated the documentation (both comments in code and under /docs)