Display a favicon for each subscription

[chuanqisun]

Here are the technical issues I'm considering. Feel free to chime in.

1. I assume we can't just point to favicon like using img url. The remote host might have CORS to block us from loading it
2. In that case, we must crawl save the favicon locally.
3. We must have some logic to update favicon when it changes. A simple solution is to always re-download it on every build.
4. We should get the correct resolution. The standard favicon.ico is 16x16. That a good starting point, but as more sites support higher resolution png formats, having those would be a nice enhancement.