Add drop_unauthorized parameter to proxy hosts

drop_unauthorized returns 444 when a client is not authorized as opposed
to 403. It can be used with Client Certificate authorization.

frontend/js/models/proxy-host.js

@@ -20,6 +20,7 @@ const model = Backbone.Model.extend({
             caching_enabled:         false,
             allow_websocket_upgrade: false,
             block_exploits:          false,
+            drop_unauthorized:       false,
             http2_support:           false,
             advanced_config:         '',
             enabled:                 true,