FEATURE: Allow embedded view to include a header (#20150)

This commits adds the ability to add a header to the embedded comments
view. One use case for this is to allow `postMessage` communication
between the comments iframe and the parent frame, for example, when
toggling the theme of the parent webpage.

app/models/theme_field.rb

@@ -337,7 +337,7 @@ class ThemeField < ActiveRecord::Base
   end
 
   def self.html_fields
-    @html_fields ||= %w[body_tag head_tag header footer after_header]
+    @html_fields ||= %w[body_tag head_tag header footer after_header embedded_header]
   end
 
   def self.scss_fields