Add links' favicon

[per-bohlin]

• Prefix the site-name with the site's favicon.
• Make the feature configurable via config/config.json: favicon: [true]|false
• extend the sites data in index.json to include the field favicon.
  If the value is the word "blank", use a transparent single-pixel image instead.
  If the value is specified, use the value instead of the default site.url + /favicon.ico
• force scale all favicon to render to the same size so the site name remain aligned