Add Web IDE dark theme feature flag

Add feature flag to support dark theme in the Web IDE

app/controllers/ide_controller.rb

@@ -6,6 +6,10 @@ class IdeController < ApplicationController
   include ClientsidePreviewCSP
   include StaticObjectExternalStorageCSP
 
+  before_action do
+    push_frontend_feature_flag(:webide_dark_theme)
+  end
+
   def index
     Gitlab::UsageDataCounters::WebIdeCounter.increment_views_count
   end