replacements: add fix script to backfill old images from Gelbooru.

Add a fix script to download images from Gelbooru for old replacements where we deleted the original
image. For archival purposes, we want to try to find the original file for every replacement.

These images will be uploaded as unposted assets under DanbooruBot's name.

script/fixes/127_download_old_replacements.rb

@@ -0,0 +1,26 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+FIX = ENV.fetch("FIX", "false").truthy?
+COND = ENV.fetch("COND", "TRUE")
+
+def download(replacement, md5)
+  url = "https://gelbooru.com/index.php?page=post&s=list&md5=#{md5}"
+  image_url = Source::Extractor.find(url).image_urls.first
+
+  upload = Upload.create!(uploader: User.system, source: image_url) if FIX && image_url.present?
+  Timeout.timeout(30) { sleep 1 until upload.reload.is_finished? } if upload
+
+  puts ({ replacement: replacement.id, upload: upload&.id, md5: md5, image_url:, duration: (upload.updated_at - upload.created_at if upload) }).to_json
+rescue Timeout::Error
+  puts ({ error: "upload timed out", replacement: replacement.id, upload: upload&.id, image_url:, }).to_json
+end
+
+PostReplacement.where(COND).find_each do |replacement|
+  old_media_asset = MediaAsset.active.find_by(md5: replacement.old_md5)
+  new_media_asset = MediaAsset.active.find_by(md5: replacement.md5)
+
+  download(replacement, replacement.old_md5) if old_media_asset.nil?
+  download(replacement, replacement.md5) if new_media_asset.nil?
+end