Fix unsafe column type change in migration (#12653)

db/migrate/20191212003415_increase_backup_size.rb

+require Rails.root.join('lib', 'mastodon', 'migration_helpers')
+
 class IncreaseBackupSize < ActiveRecord::Migration[5.2]
+  include Mastodon::MigrationHelpers
+
+  disable_ddl_transaction!
+
   def up
-    change_column :backups, :dump_file_size, :bigint
+    safety_assured do
+      change_column_type_concurrently :backups, :dump_file_size, :bigint
+      cleanup_concurrent_column_type_change :backups, :dump_file_size
+    end
   end
 
   def down
-    change_column :backups, :dump_file_size, :integer
+    safety_assured do
+      change_column_type_concurrently :backups, :dump_file_size, :integer
+      cleanup_concurrent_column_type_change :backups, :dump_file_size
+    end
   end
 end