FIX: Set unique post key for a user outside of transaction.

Previously, the Redis key was set within the transaction and the key
isn't deleted if the transaction is not successful.

Note that this isn't tested because we don't have a repro of what can
raise an error within the transaction.

https://meta.discourse.org/t/body-is-too-similar-to-what-you-previously-posted-even-when-previous-post-didnt-go-through/105436

lib/post_creator.rb

@@ -167,7 +167,6 @@ class PostCreator
         create_topic
         save_post
         extract_links
-        store_unique_post_key
         track_topic
         update_topic_stats
         update_topic_auto_close
@@ -182,6 +181,7 @@ class PostCreator
     end
 
     if @post && errors.blank? && !@opts[:import_mode]
+      store_unique_post_key
       # update counters etc.
       @post.topic.reload