FEATURE: Staff members can lock posts

Locking a post prevents it from being edited. This is useful if the user
has posted something which has been edited out, and the staff members don't
want them to be able to edit it back in again.

app/models/user_history.rb

@@ -63,7 +63,9 @@ class UserHistory < ActiveRecord::Base
                           backup_download: 45,
                           backup_destroy: 46,
                           notified_about_get_a_room: 47,
-                          change_name: 48)
+                          change_name: 48,
+                          post_locked: 49,
+                          post_unlocked: 50)
   end
 
   # Staff actions is a subset of all actions, used to audit actions taken by staff users.
@@ -104,7 +106,9 @@ class UserHistory < ActiveRecord::Base
                         :activate_user,
                         :change_readonly_mode,
                         :backup_download,
-                        :backup_destroy]
+                        :backup_destroy,
+                        :post_locked,
+                        :post_unlocked]
   end
 
   def self.staff_action_ids