adds uniqueness validations for transactions

Per our conversation with Niels, I added a validation on the Transaction model that checks for uniqueness of new transactions based on the transaction_code scoped by location_id and transaction_time. This means that the validation will only check transactions that have have the same location_id and transaction_time when checking for uniqueness of the transaction_code. I also added model and controller tests to ensure that that this is functioning as expected with the Transaction Controller.
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