ActivityUpdaterJob->after_action: fix txn conflict

Creating a post would update thredded user detail (incr post count),
but so does ActivityUpdaterJob (update last_seen_at).

If these happen the same time on SQLite3, the following error is thrown:

  SQLite3::BusyException: database is locked

Moving the schedule to after_action prevents this.

app/controllers/thredded/posts_controller.rb

@@ -4,7 +4,7 @@ class PostsController < Thredded::ApplicationController
     include ActionView::RecordIdentifier
 
     helper_method :topic
-    before_action :update_user_activity
+    after_action :update_user_activity
 
     after_action :verify_authorized