Add a user_seen DiscourseEvent for plugins to hook into

app/models/user.rb

@@ -521,6 +521,8 @@ class User < ActiveRecord::Base
     # using update_column to avoid the AR transaction
     update_column(:last_seen_at, now)
     update_column(:first_seen_at, now) unless self.first_seen_at
+
+    DiscourseEvent.trigger(:user_seen, self)
   end
 
   def self.gravatar_template(email)