Use the rss item's url when handling relative image links (#15497)

* Use the feed source url when the feed url (from links) is empty

This probably should just be using @feed_source_url always to make a
base path for absolute paths without a hostname, rather than expecting
this from the feed.

An example why we should not be using @feed.url would be
individual posts at domain/slug but the "index" page at
domain/blog (with a generic screen on /), and images hosted from
/assets/ without /blog/ anywhere in the asset path.

Partial fix for #15488

* Use item.url as base_url for relative links

It's possible the rss feed and the contained articles are served from
separate hosts/subdomains/paths. Don't assume the entries are related
to the feed or the feed_source_url, use the item's url for relative
links in that item.

* Update failing spec to add url to item double

app/services/feeds/assemble_article_markdown.rb

@@ -31,6 +31,13 @@ module Feeds
 
     private
 
+    def base_url
+      @base_url ||=
+        URI.parse(@item.url).then do |uri|
+          "#{uri.scheme}://#{uri.host}"
+        end
+    end
+
     def processed_title
       @title.truncate(128, omission: "...", separator: " ")
     end
@@ -43,7 +50,7 @@ module Feeds
 
     def assemble_body_markdown
       cleaned_content = Feeds::CleanHtml.call(get_content)
-      cleaned_content = thorough_parsing(cleaned_content, @feed.url)
+      cleaned_content = thorough_parsing(cleaned_content, base_url)
 
       content = ReverseMarkdown
         .convert(cleaned_content, github_flavored: true)

spec/services/feeds/assemble_article_markdown_spec.rb

@@ -18,6 +18,7 @@ RSpec.describe Feeds::AssembleArticleMarkdown, type: :service do
       categories: %w[tag1 tag2 tag3 tag4 tag5],
       published: "2020-12-20",
       content: content,
+      url: "https://feed.source",
     )
   end