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.

plugins/styleguide/plugin.rb

@@ -15,7 +15,7 @@ Discourse::Application.routes.append do
 end
 
 after_initialize do
-  register_asset_filter do |type, request|
-    request&.fullpath&.start_with?('/styleguide')
+  register_asset_filter do |type, request, opts|
+    (opts[:path] || '').start_with?('/styleguide')
   end
 end