twitter: fix preview_urls when source url is a direct image.

Fix preview_urls returning an empty array when the source url is a
direct image from Twitter.

Also return preview_urls in /source.json.

app/logical/sources/strategies/base.rb

@@ -269,6 +269,8 @@ module Sources
           :artists => artists.as_json(include: :sorted_urls),
           :image_url => image_url,
           :image_urls => image_urls,
+          :preview_url => preview_url,
+          :preview_urls => preview_urls,
           :page_url => page_url,
           :canonical_url => canonical_url,
           :normalized_for_artist_finder_url => normalize_for_artist_finder,

app/logical/sources/strategies/twitter.rb

@@ -80,10 +80,14 @@ module Sources::Strategies
     end
 
     def preview_urls
-      return image_urls if api_response.blank?
-
-      api_response.dig(:extended_entities, :media).to_a.map do |media|
-        media[:media_url_https] + ":small"
+      if api_response.dig(:extended_entities, :media).present?
+        api_response.dig(:extended_entities, :media).to_a.map do |media|
+          media[:media_url_https] + ":small"
+        end
+      else
+        image_urls.map do |url|
+          url.gsub(/:orig\z/, ":small")
+        end
       end
     end