danbooru::http: factor out Cloudflare Polish bypassing.

* Factor out the Cloudflare Polish bypass code to a standalone feature.

* Add `http_downloader` method to the base source strategy. This is a
  HTTP client that should be used for downloading images or making
  requests to images. This client ensures that referrer spoofing and
  Cloudflare bypassing are performed.

This fixes a bug with the upload page reporting the polished filesize
instead of the original filesize when uploading ArtStation images.

app/logical/danbooru/http/unpolish_cloudflare.rb

@@ -0,0 +1,20 @@
+# Bypass Cloudflare Polish (https://support.cloudflare.com/hc/en-us/articles/360000607372-Using-Cloudflare-Polish-to-compress-images)
+
+module Danbooru
+  class Http
+    class UnpolishCloudflare < HTTP::Feature
+      HTTP::Options.register_feature :unpolish_cloudflare, self
+
+      def perform(request, &block)
+        response = yield request
+
+        if response.headers["CF-Polished"].present?
+          request.uri.query_values = request.uri.query_values.to_h.merge(danbooru_no_polish: SecureRandom.uuid)
+          response = yield request
+        end
+
+        response
+      end
+    end
+  end
+end