Add author name to Atom feed previews.
In browser preview, explicitly say if a feed is empty.
Fix the bug that I tried to fix in b49ecd720c725f687d8026d8111b3dd39c6d4f99 properly this time. Had forgotten that not all validate events occur on the toolbar menu button, they can occur on menu items too (and my last 'fix' caused an error when they did so).
Pull YouTube functionality into its own .js file, which is loaded dynamically only when on a YouTube page.

Now finds and puts RSS feeds for the user's/channel's subscriptions and favourites in the unsorted section of the menu, and  feeds for their playlists in the sorted section. The sorted section now has a menu header (given to us when we requested the feeds via the YouTube API) as well as  the separator.
Move all the .js files into a js subdirectory, to keep things tidier.