update.py: For apps, add links to flathub and github
    
    Add a field that includes links to, e.g.:
      https://flathub.org/apps/details/com.spotify.Client
      https://github.com/flathub/com.spotify.Client
    
    Letting people working on reviewing apps not have to make these URLs
    themselves. We can't easily generate these links for extensions, so
    stick to apps for now.