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/danbooru/http.rb

@@ -117,6 +117,14 @@ module Danbooru
       use(cache: { expires_in: expires_in })
     end
 
+    def proxy(host: Danbooru.config.http_proxy_host, port: Danbooru.config.http_proxy_port.to_i, username: Danbooru.config.http_proxy_username, password: Danbooru.config.http_proxy_password)
+      return self if host.blank?
+
+      dup.tap do |o|
+        o.http = o.http.via(host, port, username, password)
+      end
+    end
+
     # allow requests only to public IPs, not to local or private networks.
     def public_only
       dup.tap do |o|