Add feeds favicon

[ai]

• Add algorithm to find favicon. We can use good npm library or write our own.
  • We should load URL and check for icons
  • If the URL is not HTML we should load /
  • In HTML we should prefer type="image/svg+xml" rel="icon" (we should be ready that rel contains multiple words)
  • If not, we can use the first rel="icon"
  • If there is no icons, let’s check that /favicon.ico exists
• In preview we need a favicon before user will add feed, so we need to adds favicons to $candidates (or create another store).
• We need to add icon key to FeedValue
• We need to detect and set icon in addFeed() (if feedValue.icon is not set already).
• We need to be ready when icon will be updated. I suggest add updateFeedIcon(feedId) and create some <UiFeedIcon> component, which will call updateFeedIcon() on <img>.onerror.