Upload preview: use normalized image url

This means that the preview will work even when the user doesn't use the
direct image link. e.g. The user can use the bookmarklet on Pixiv
mode=medium pages.

app/controllers/uploads_controller.rb

@@ -8,6 +8,15 @@ class UploadsController < ApplicationController
     @upload = Upload.new(:rating => "q")
     if params[:url]
       @post = Post.find_by_source(params[:url])
+
+      @normalized_url = params[:url]
+      headers = {
+        "User-Agent" => "#{Danbooru.config.safe_app_name}/#{Danbooru.config.version}"
+      }
+      Downloads::Strategies::Base.strategies.each do |strategy|
+        @normalized_url, headers = strategy.new.rewrite(@normalized_url, headers)
+      end
+
       begin
         @source = Sources::Site.new(params[:url])
       rescue Exception

app/views/uploads/_image.html.erb

@@ -1,5 +1,5 @@
 <% if params[:url] %>
-  <%= image_tag(params[:url], :title => "Preview", :id => "image") %>
+  <%= image_tag(@normalized_url, :title => "Preview", :id => "image") %>
   <% if params[:ref] %>
     <p><strong>Referrer: </strong> <%= params[:ref] %></p>
   <% end %>