Bug: the uploads page showed a remote size of 146 bytes for Pixiv uploads.
Cause: we didn't spoof the Referer header when making the HEAD request
for the image, causing Pixiv to return a 403 error.
Also fix the case where the Content-Length header is absent.
Remove the Downloads::File class. Move download methods to
Danbooru::Http instead. This means that:
* HTTParty has been replaced with http.rb for downloading files.
* Downloading is no longer tightly coupled to source strategies. Before
Downloads::File tried to automatically look up the source and download
the full size image instead if we gave it a sample url. Now we can
do plain downloads without source strategies altering the url.
* The Cloudflare Polish check has been changed from checking for a
Cloudflare IP to checking for the CF-Polished header. Looking up the
list of Cloudflare IPs was slow and flaky during testing.
* The SSRF protection code has been factored out so it can be used for
normal http requests, not just for downloads.
* The Webmock gem can be removed, since it was only used for stubbing
out certain HTTParty requests in the download tests. The Webmock gem
is buggy and caused certain tests to fail during CI.
* The retriable gem can be removed, since we no longer autoretry failed
downloads. We assume that if a download fails once then retrying
probably won't help.
* On the /uploads/new page, instead of just showing a "This post has
probably already been uploaded" message, show the actual thumbnails of
posts having the same source as what the user is trying to upload.
* Move the iqdb results section up top, beside the related posts section.
Also makes it so that using the bookmarklet always triggers async upload
preprocessing. Before it was only triggered when the source passed a
dupe check, but that check was inaccurate (#3873).
