* 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.
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