Set channel thumbnail as icon for feeds (#1106)

src/invidious.cr

@@ -1544,6 +1544,7 @@ get "/feed/channel/:ucid" do |env|
       xml.element("link", rel: "self", href: "#{HOST_URL}#{env.request.resource}")
       xml.element("id") { xml.text "yt:channel:#{channel.ucid}" }
       xml.element("yt:channelId") { xml.text channel.ucid }
+      xml.element("icon") { xml.text channel.author_thumbnail }
       xml.element("title") { xml.text channel.author }
       xml.element("link", rel: "alternate", href: "#{HOST_URL}/channel/#{channel.ucid}")