Automatic feed channel icons feature for xExtension-YouTube

[Inverle]

Dependent on FreshRSS/FreshRSS#7646

Right now, the icon only gets set after the feed is added.
(the correct setting must be enabled in order for the icons to be downloaded)

TODO:

• redownload icon whenever needed
• add a button to set the icon for individual feeds (needs work in Implement custom feed favicons FreshRSS#7646)
• add option to download icons for all of the channel feeds
  • show fetching progress and make individual requests for each feed with JS so that PHP doesn't timeout (necessary when there's a lot of feeds)
• add option to reset all of the channel icons back to the YT icon
• fix tests