New bootstrap.json endpoint for starting up Discourse

Discourse needs a bunch of data preloaded before it can start up.
Normally we throw blobs of this into the HTML document that is requested
but in some cases that's awkward to retrieve.

For example with Ember CLI you have a separate javascript application
that needs to make its own HTML.

This API endpoint returns a JSON object with all the data Discourse needs to
bootstrap and start up.

app/controllers/bootstrap_controller.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+class BootstrapController < ApplicationController
+  include ApplicationHelper
+  include ActionView::Helpers::AssetUrlHelper
+
+  # This endpoint allows us to produce the data required to start up Discourse via JSON API,
+  # so that you don't have to scrape the HTML for `data-*` payloads
+  def index
+    locale = script_asset_path("locales/#{I18n.locale}")
+
+    preload_anonymous_data
+    if current_user
+      current_user.sync_notification_channel_position
+      preload_current_user_data
+    end
+
+    bootstrap = {
+      theme_ids: theme_ids,
+      title: SiteSetting.title,
+      current_homepage: current_homepage,
+      locale_script: "#{Discourse.base_url}#{locale}",
+      setup_data: client_side_setup_data,
+      preloaded: @preloaded
+    }
+
+    render_json_dump(bootstrap: bootstrap)
+  end
+end