users: add ability to undelete accounts.

Add ability to undelete accounts from within the console. Their password is reset, their name is
restored to their last known user name, and a mod action is logged.

app/models/mod_action.rb

@@ -17,6 +17,7 @@ class ModAction < ApplicationRecord
   # Misc:     6-19
   enum category: {
     user_delete: 2,
+    user_undelete: 3,
     user_ban: 4,
     user_unban: 5,
     user_name_change: 6,

app/models/user_event.rb

@@ -15,6 +15,7 @@ class UserEvent < ApplicationRecord
     logout: 100,
     user_creation: 200,
     user_deletion: 300,
+    user_undeletion: 310,
     password_reset: 400,
     password_change: 500,
     email_change: 600,