DEV: Drop ember-cli-based SCSS and locale compilation (#31407)

This totally separate SCSS and i18n compilation pipelines only existed
so that we could run `ember exam` in CI without starting Rails.

Now that our CI has such heavy caching of Ruby dependencies and database
migrations, the speed benefit of this is not worth the cost of
maintaining these separate pipelines.

Therefore, this commit removes that system, and updates CI to use
`bin/rake qunit:test`. That will start up a Rails server and proxy
stylesheet/locale requests to it. This strategy was already used for our
theme and plugin qunit test runs.

app/controllers/bootstrap_controller.rb

@@ -4,16 +4,26 @@ class BootstrapController < ApplicationController
   skip_before_action :redirect_to_login_if_required, :check_xhr
 
   def plugin_css_for_tests
+    targets = Discourse.find_plugin_css_assets(include_disabled: true, desktop_view: true)
+    render_css_for_tests(targets)
+  end
+
+  def core_css_for_tests
+    targets = %w[color_definitions desktop admin]
+    render_css_for_tests(targets)
+  end
+
+  private
+
+  def render_css_for_tests(targets)
     urls =
-      Discourse
-        .find_plugin_css_assets(include_disabled: true, desktop_view: true)
-        .map do |target|
-          details = Stylesheet::Manager.new().stylesheet_details(target, "all")
-          details[0][:new_href]
-        end
+      targets.map do |target|
+        details = Stylesheet::Manager.new().stylesheet_details(target, "all")
+        details[0][:new_href]
+      end
 
     stylesheet = <<~CSS
-      /* For use in tests only - `@import`s all plugin stylesheets */
+      /* For use in tests only */
       #{urls.map { |url| "@import \"#{url}\";" }.join("\n")}
     CSS