Disable STI for BackupLabel to prefil type correctly

When ProjectLabel is migrated into backup_labels table,
we want type column to have "ProjectLabel" value.

In Rails 6.1 STI behavior changed so the type column
get prefilled with BackupLabel value instead

db/post_migrate/20200305082754_remove_duplicate_labels_from_project.rb

@@ -9,6 +9,8 @@ class RemoveDuplicateLabelsFromProject < ActiveRecord::Migration[6.0]
   disable_ddl_transaction!
 
   class BackupLabel < Label
+    self.inheritance_column = :_type_disabled
+
     self.table_name = 'backup_labels'
   end