FIX: Ember CLI was losing some preloaded data (#13406)

The `bootstrap.json` contains most preloaded information but some routes
provide extra information, such as invites.

This fixes the issue by having the preload request pass on the preloaded
data from the source page, which is then merged with the bootstrap's
preloaded data for the final HTML payload.

app/controllers/application_controller.rb

@@ -322,7 +322,9 @@ class ApplicationController < ActionController::Base
   end
 
   def send_ember_cli_bootstrap
-    head 200, content_type: "text/html", "X-Discourse-Bootstrap-Required": true
+    response.headers['X-Discourse-Bootstrap-Required'] = true
+    response.headers['Content-Type'] = "application/json"
+    render json: { preloaded: @preloaded }
   end
 
   # If a controller requires a plugin, it will raise an exception if that plugin is