DEV: Improve Ember CLI's bootstrap logic (#12792)

* DEV: Give a nicer error when `--proxy` argument is missing

* DEV: Improve Ember CLI's bootstrap logic

Instead of having Ember CLI know which URLs to proxy or not, have it try
the URL with a special header `HTTP_X_DISCOURSE_EMBER_CLI`. If present,
and Discourse thinks we should bootstrap the application, it will
instead stop rendering and return a HTTP HEAD with a response header
telling Ember CLI to bootstrap.

In other words, any time Rails would otherwise serve up the HTML for the
Ember app, it stops and says "no, you do it."

* DEV: Support asset filters by path using a new options object

Without this, Ember CLI's bootstrap would not get the assets it wants
because the path it was requesting was different than the browser path.
This adds an optional request header to fix it.

So far this is only used by the styleguide.

app/controllers/application_controller.rb

@@ -107,9 +107,23 @@ class ApplicationController < ActionController::Base
 
   class RenderEmpty < StandardError; end
   class PluginDisabled < StandardError; end
+  class EmberCLIHijacked < StandardError; end
+
+  def catch_ember_cli_hijack
+    yield
+  rescue ActionView::Template::Error => ex
+    raise ex unless ex.cause.is_a?(EmberCLIHijacked)
+    send_ember_cli_bootstrap
+  end
 
   rescue_from RenderEmpty do
-    with_resolved_locale { render 'default/empty' }
+    catch_ember_cli_hijack do
+      with_resolved_locale { render 'default/empty' }
+    end
+  end
+
+  rescue_from EmberCLIHijacked do
+    send_ember_cli_bootstrap
   end
 
   rescue_from ArgumentError do |e|
@@ -286,13 +300,19 @@ class ApplicationController < ActionController::Base
       rescue Discourse::InvalidAccess
         return render plain: message, status: status_code
       end
-      with_resolved_locale do
-        error_page_opts[:layout] = opts[:include_ember] ? 'application' : 'no_ember'
-        render html: build_not_found_page(error_page_opts)
+      catch_ember_cli_hijack do
+        with_resolved_locale do
+          error_page_opts[:layout] = opts[:include_ember] ? 'application' : 'no_ember'
+          render html: build_not_found_page(error_page_opts)
+        end
       end
     end
   end
 
+  def send_ember_cli_bootstrap
+    head 200, content_type: "text/html", "X-Discourse-Bootstrap-Required": true
+  end
+
   # If a controller requires a plugin, it will raise an exception if that plugin is
   # disabled. This allows plugins to be disabled programatically.
   def self.requires_plugin(plugin_name)