Add username template variables to Welcome User email template (#20352)

* Add username and name_or_username variables to SystemMessage defaults

* Allow username and name variables on welcome_user email template overrides

* Satisfy linting

* Add test

app/models/translation_override.rb

@@ -35,6 +35,7 @@ class TranslationOverride < ActiveRecord::Base
       optional_cat
       optional_tags
     ],
+    %w[system_messages.welcome_user] => %w[username name name_or_username],
   }
 
   include HasSanitizableFields