Replace hyphen with underscore in Mastodon address

Unfortunately, Mastodon only allows underscores for usernames, and
reversely, akkounts only allows hyphens and no underscores.

app/models/user.rb

@@ -131,7 +131,7 @@ class User < ApplicationRecord
 
   def mastodon_address
     return nil unless Setting.mastodon_enabled?
-    "#{self.cn}@#{Setting.mastodon_address_domain}"
+    "#{self.cn.gsub("-", "_")}@#{Setting.mastodon_address_domain}"
   end
 
   def valid_attribute?(attribute_name)