[FR]: Add "Discover feeds" dialog

[martinrotter]

Brief description of the feature request

FOR ME:

• Integrate into "add dialog" via "StackPanel"
• Dialog will enter URL of any website and target parent node, RSS Guard will find all "feeds" on website:
  • standard RSS/ATOM/JSON via HTML head or directly "/feed" "/rss"
  • sitemap and sitemap indexes via robots.txt file or directly "/sitemap" "sitemap.xml"
  • Youtube channel feeds if youtube channel URL is entered
• After detection list will appear below url with checkboxes
• if user selects one, there will be option to go "advanced" which will switch to classic add feed page
• if user selects more, there will be only option "Import" (which will import just guessed feeds)
• all checked will be added to selected parent. Then dialog closes.