FEATURE: Update the topic excerpt when the OP is rebaked (#9852)

* We now have a site setting "topic_excerpt_maxlength" that is used when the OP is created or revised to generate a topic excerpt.
* However, posts created before this setting was introduced cannot benefit from this change unless they are revised, and if the topic excerpt length setting is changed that situation is also not covererd.
* This PR makes a change to rebake! to update the topic excerpt IF the post is the OP.

app/models/topic.rb

@@ -1476,6 +1476,13 @@ class Topic < ActiveRecord::Base
     private_topic
   end
 
+  def update_excerpt(excerpt)
+    update_column(:excerpt, excerpt)
+    if archetype == "banner"
+      ApplicationController.banner_json_cache.clear
+    end
+  end
+
   def pm_with_non_human_user?
     sql = <<~SQL
     SELECT 1 FROM topics