Add HTTP proxy support.

Add support for using a proxy for HTTP requests. Only used for external
requests, such as downloading files or talking to source sites such as
Pixiv or Twitter, not for internal requests, such as talking to IQDB or
Reportbooru.

app/logical/sources/strategies/pixiv.rb

@@ -269,7 +269,7 @@ module Sources
       end
 
       def api_client
-        PixivAjaxClient.new(Danbooru.config.pixiv_phpsessid)
+        PixivAjaxClient.new(Danbooru.config.pixiv_phpsessid, http: http)
       end
 
       def api_illust