Ensure `Migration.new` is also included in the transaction

Sort of a weird edge case, but one could theoretically perform db operations inside `initializer` and that wouldn't be rolled back on failures.

We need to ensure that [.new](https://github.com/thoughtbot/data_customs/blob/e8e76ed14d99af30ed49235ac43778f29c9d3249/lib/data_customs/migration.rb#L8) is inside the transaction block. I'd probably move the transaction to `self.run`.