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.
* Switch CloudflareService from HttpartyCache to Danbooru::Http.
* Purge cached urls from Cloudflare when a post is replaced and the md5
doesn't change. This happens when a corrupted image is replaced or
thumbnails are regenerated. Before we purged urls when a post was
expunged, which was unneeded because those urls can expire naturally.
It was also wrong because the subdomains were hardcoded, the urls used
http:// instead of https://, and we didn't account for tagged urls.
