Override voucher name field to avoid django-oscar's unique constraint

[mumarkhan999]

⛔️ DEPRECATION WARNING

This repository is deprecated and in maintainence-only operation while we work on a replacement, please see this announcement for more information.

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Required Testing

• Before deploying this change, complete a purchase in the stage environment.
  (^ We can remove that manual check once REV-2624 is done and the corresponding e2e test runs again)

Description

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[mumarkhan999]

Closing as this PR is not needed.