sources: remove image_url method from base strategy.

Remove the `image_url` method from source strategies. This method would
return only the first image if a source had multiple images. The
`image_urls` method should be used instead. Tests were the main place
that still used `image_url` instead of `image_urls`.

Also make post replacements return an error if replacing with a source
that contains multiple images, instead of just blindly replacing the
post with the first image in the source.

app/logical/post_replacement_processor.rb

@@ -67,6 +67,9 @@ class PostReplacementProcessor
     strategy = Sources::Strategies.find(source_url, referer_url)
     raise NotImplementedError, "No login credentials configured for #{strategy.site_name}." unless strategy.class.enabled?
 
-    strategy.download_file!
+    image_urls = strategy.image_urls
+    raise "#{source_url} contains multiple images" if image_urls.size > 1
+
+    strategy.download_file!(image_urls.first)
   end
 end

app/logical/sources/strategies/base.rb

@@ -129,10 +129,6 @@ module Sources
         []
       end
 
-      def image_url
-        image_urls.first
-      end
-
       # A smaller representation of the image that's suitable for
       # displaying previews.
       def preview_urls
@@ -154,7 +150,7 @@ module Sources
       # 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_url
+        page_url || image_urls.first
       end
 
       # A name to suggest as the artist's tag name when creating a new artist.
@@ -194,8 +190,11 @@ module Sources
         nil
       end
 
-      # Download the file at the given url, or at the main image url by default.
-      def download_file!(download_url = image_url)
+      # Download the file at the given url. Raises Danbooru::Http::DownloadError if the download fails, or
+      # Danbooru::Http::FileTooLargeError if the file is too large.
+      #
+      # @return [MediaFile] the downloaded file
+      def download_file!(download_url)
         response, file = http_downloader.download_media(download_url)
         file
       end
@@ -310,7 +309,6 @@ module Sources
             :profile_urls => profile_urls
           },
           :artists => artists.as_json(include: :sorted_urls),
-          :image_url => image_url,
           :image_urls => image_urls,
           :preview_url => preview_url,
           :preview_urls => preview_urls,

app/logical/sources/strategies/hentai_foundry.rb

@@ -58,7 +58,7 @@ module Sources
       end
 
       def canonical_url
-        image_url
+        image_urls.first
       end
 
       def profile_url

app/logical/sources/strategies/mastodon.rb

@@ -18,10 +18,6 @@ module Sources::Strategies
       end
     end
 
-    def image_url
-      image_urls.first
-    end
-
     def image_urls
       if parsed_url.image_url?
         [parsed_url.full_image_url]

app/logical/sources/strategies/moebooru.rb

@@ -27,7 +27,7 @@ module Sources
       end
 
       def canonical_url
-        image_url
+        image_urls.first
       end
 
       def normalize_for_source

app/logical/sources/strategies/null.rb

@@ -12,7 +12,7 @@ module Sources
       end
 
       def canonical_url
-        image_url
+        url
       end
 
       def normalize_for_source

app/logical/sources/strategies/pixiv.rb

@@ -62,7 +62,7 @@ module Sources
       end
 
       def canonical_url
-        image_url
+        image_urls.first
       end
 
       def profile_url
@@ -122,7 +122,7 @@ module Sources
         tag.gsub(/\d+users入り\z/i, "")
       end
 
-      def download_file!(url = image_url)
+      def download_file!(url)
         file = super(url)
         file.frame_data = ugoira_frame_data if is_ugoira?
         file