sources: remove canonical_url method.

Refactor source strategies to remove the `canonical_url` method.

`canonical_url` returned the URL that should be used as the source of
the post after upload. Now we simply use `Source::URL#page_url` to
determine the source after upload. If the source is an image URL that is
convertible to a page URL, then the image URL is used as the source. If
the source is an image URL that is not convertible to a page URL, then
the page URL is used as the source.

This simplifies source strategies so that all they have to care about is
implementing the `Source::URL#page_url` and `Sources::Strategies#page_url`
methods, and the preferred source will be chosen for posts automatically.

app/logical/sources/strategies.rb

@@ -31,9 +31,5 @@ module Sources
       strategy = all.lazy.map { |s| s.new(url, referer) }.detect(&:match?)
       strategy || default&.new(url, referer)
     end
-
-    def self.canonical(url, referer)
-      find(url, referer).canonical_url
-    end
   end
 end

app/logical/sources/strategies/base.rb

@@ -65,20 +65,23 @@ module Sources
         []
       end
 
-      # Whatever <tt>url</tt> is, this method should return a link to the HTML
-      # page containing the resource. It should not be a binary file. It will
-      # eventually be assigned as the source for the post, but it does not
-      # represent what the downloader will fetch.
+      # The URL of the page containing the image, or nil if it can't be found.
+      #
+      # The source of the post will be set to the page URL if it's not possible
+      # to convert the image URL to a page URL for this site.
+      #
+      # For example, for sites like Twitter and Tumblr, it's not possible to
+      # convert image URLs to page URLs, so the page URL will be used as the
+      # source for these sites. For sites like Pixiv and DeviantArt, it is
+      # possible to convert image URLs to page URLs, so the image URL will be
+      # used as the source for these sites. This is determined by whether
+      # `Source::URL#page_url` returns a URL or nil.
+      #
+      # @return [String, nil]
       def page_url
         nil
       end
 
-      # This will be the url stored in posts. Typically this is the page
-      # url, but on some sites it may be preferable to store the image url.
-      def canonical_url
-        page_url || image_urls.first
-      end
-
       # A name to suggest as the artist's tag name when creating a new artist.
       # This should usually be the artist's account name.
       def tag_name
@@ -197,7 +200,7 @@ module Sources
       # uploaded from the same source. These may be duplicates, or they may be
       # other posts from the same gallery.
       def related_posts_search_query
-        "source:#{canonical_url}"
+        "source:#{url}"
       end
 
       def related_posts(limit = 5)
@@ -222,7 +225,6 @@ module Sources
           :artists => artists.as_json(include: :sorted_urls),
           :image_urls => image_urls,
           :page_url => page_url,
-          :canonical_url => canonical_url,
           :tags => tags,
           :normalized_tags => normalized_tags,
           :translated_tags => translated_tags,

app/logical/sources/strategies/hentai_foundry.rb

@@ -47,10 +47,6 @@ module Sources
         parsed_url.username || parsed_referer&.username
       end
 
-      def canonical_url
-        image_urls.first
-      end
-
       def profile_url
         return nil if artist_name.blank?
         "https://www.hentai-foundry.com/user/#{artist_name}"

app/logical/sources/strategies/moebooru.rb

@@ -21,10 +21,6 @@ module Sources
         "https://#{domain}/post/show/#{post_id}"
       end
 
-      def canonical_url
-        image_urls.first
-      end
-
       def tags
         api_response[:tags].to_s.split.map do |tag|
           [tag, "https://#{domain}/post?tags=#{CGI.escape(tag)}"]

app/logical/sources/strategies/newgrounds.rb

@@ -75,15 +75,6 @@ module Sources
         DText.from_html(artist_commentary_desc)
       end
 
-      # The image url should be the post source, if we can generate the page url from the image url.
-      def canonical_url
-        if page_url.present?
-          url
-        else
-          page_url
-        end
-      end
-
       def user_name
         parsed_url.username || parsed_referer&.username
       end

app/logical/sources/strategies/null.rb

@@ -11,10 +11,6 @@ module Sources
         nil
       end
 
-      def canonical_url
-        url
-      end
-
       def artists
         ArtistFinder.find_artists(url)
       end

app/logical/sources/strategies/pixiv.rb

@@ -57,10 +57,6 @@ module Sources
         "https://www.pixiv.net/artworks/#{illust_id}"
       end
 
-      def canonical_url
-        image_urls.first
-      end
-
       def profile_url
         if api_illust[:userId].present?
           "https://www.pixiv.net/users/#{api_illust[:userId]}"
@@ -128,7 +124,7 @@ module Sources
       end
 
       def related_posts_search_query
-        illust_id.present? ? "pixiv:#{illust_id}" : "source:#{canonical_url}"
+        illust_id.present? ? "pixiv:#{illust_id}" : "source:#{url}"
       end
 
       def is_ugoira?