iqdb: download files inside danbooru, not inside iqdb.

Instead of sending IQDB the image url and letting it download the file,
download the file on Danbooru's end and send IQDB the downloaded file.

This fixes several issues:

* Some sites need the referer header set to avoid hotlink protection
  when downloading the file. Danbooru knows how to deal with this but
  IQDB doesn't.

* We need to enforce certain restrictions when downloading files,
  including setting max filesize limits, setting max timeouts, and not
  allowing downloads from forbidden IPs (to avoid SSRF attacks).
  Danbooru knows how to handle these things but IQDB doesn't.

app/logical/iqdb_proxy.rb

@@ -5,6 +5,12 @@ class IqdbProxy
     Danbooru.config.iqdbs_server.present?
   end
 
+  def self.download(url, type)
+    download = Downloads::File.new(url)
+    file, strategy = download.download!(url: download.send(type))
+    file
+  end
+
   def self.search(params)
     raise NotImplementedError, "the IQDBs service isn't configured" unless enabled?
 
@@ -13,13 +19,14 @@ class IqdbProxy
     high_similarity = params[:high_similarity]&.to_f&.clamp(0.0, 100.0) || 65.0
 
     if params[:file].present?
-      results = query(file: params[:file], limit: limit)
+      file = params[:file]
+      results = query(file: file, limit: limit)
     elsif params[:url].present?
-      url = Sources::Strategies.find(params[:url]).preview_url
-      results = query(url: url, limit: limit)
+      file = download(params[:url], :preview_url)
+      results = query(file: file, limit: limit)
     elsif params[:image_url].present?
-      url = Sources::Strategies.find(params[:image_url]).image_url
-      results = query(url: url, limit: limit)
+      file = download(params[:image_url], :image_url)
+      results = query(file: file, limit: limit)
     elsif params[:post_id].present?
       url = Post.find(params[:post_id]).preview_file_url
       results = query(url: url, limit: limit)
@@ -32,6 +39,8 @@ class IqdbProxy
     high_similarity_matches, low_similarity_matches = matches.partition { |match| match["score"] >= high_similarity }
 
     [high_similarity_matches, low_similarity_matches, matches]
+  ensure
+    file.try(:close)
   end
 
   def self.query(params)