move topic excerpt code to one method to DRY it up and for extensibility

app/models/post.rb

@@ -406,6 +406,10 @@ class Post < ActiveRecord::Base
     Post.excerpt(cooked, maxlength, options)
   end
 
+  def excerpt_for_topic
+    Post.excerpt(cooked, 220, strip_links: true)
+  end
+
   def is_first_post?
     post_number.blank? ?
       topic.try(:highest_post_number) == 0 :

lib/post_creator.rb

@@ -397,7 +397,7 @@ class PostCreator
       attrs[:last_posted_at] = @post.created_at
       attrs[:last_post_user_id] = @post.user_id
       attrs[:word_count] = (@topic.word_count || 0) + @post.word_count
-      attrs[:excerpt] = @post.excerpt(220, strip_links: true) if new_topic?
+      attrs[:excerpt] = @post.excerpt_for_topic if new_topic?
       attrs[:bumped_at] = @post.created_at unless @post.no_bump
       attrs[:updated_at] = 'now()'
       @topic.update_columns(attrs)

lib/post_revisor.rb

@@ -533,7 +533,7 @@ class PostRevisor
   end
 
   def update_topic_excerpt
-    excerpt = @post.excerpt(220, strip_links: true)
+    excerpt = @post.excerpt_for_topic
     @topic.update_column(:excerpt, excerpt)
     if @topic.archetype == "banner"
       ApplicationController.banner_json_cache.clear