Allow existing user records with reserved usernames to be saved

app/models/user.rb

@@ -38,7 +38,8 @@ class User < ApplicationRecord
                            message: "is invalid. Usernames need to start with a letter."
   # FIXME This needs a server restart to apply values
   validates_format_of :cn, without: /\A(#{Setting.reserved_usernames.join('|')})\z/i,
-                           message: "has already been taken"
+                           message: "has already been taken",
+                           unless: Proc.new{ |u| u.persisted? }
 
   validates_uniqueness_of :email
   validates :email, email: true