Show site favicon

If site have a favicon, change the colored ball to the favicon in the sites list.

Start by locating the sites-list entry point and the code that renders the colored ball, then inspect how each site's favicon can be identified. Done means sites with a favicon show it in the list, while sites without one retain the colored ball; add or update the relevant frontend test if the project has one.

索引モデルが issue の本文から書いたものです。