Set the default User-Agent sent for outgoing http requests.

* Add a `Danbooru.config.http_headers` config option.

* Refactor various places to use this option instead of setting the user
  agent manually.

app/logical/image_proxy.rb

@@ -16,11 +16,7 @@ class ImageProxy
       raise "Proxy not allowed for this site"
     end
 
-    headers = {
-      "Referer" => fake_referer_for(url),
-      "User-Agent" => "#{Danbooru.config.safe_app_name}/#{Danbooru.config.version}"
-    }
-    response = HTTParty.get(url, Danbooru.config.httparty_options.merge(headers: headers))
+    response = HTTParty.get(url, Danbooru.config.httparty_options.deep_merge(headers: {"Referer" => fake_referer_for(url)}))
     if response.success?
       return response
     else