Add a message to the staff logs indicating why a user was purged

app/models/user.rb

@@ -737,7 +737,7 @@ class User < ActiveRecord::Base
     destroyer = UserDestroyer.new(Discourse.system_user)
     to_destroy.each do |u|
       begin
-        destroyer.destroy(u)
+        destroyer.destroy(u, context: I18n.t(:purge_reason))
       rescue Discourse::InvalidAccess
         # if for some reason the user can't be deleted, continue on to the next one
       end

config/locales/server.en.yml

@@ -35,6 +35,8 @@ en:
   via: "%{username} via %{site_name}"
   is_reserved: "is reserved"
 
+  purge_reason: "Automatically deleted due to being old and unverified"
+
   errors:
     messages:
       too_long_validation: "is limited to %{max} characters; you entered %{length}."