Django-oscar

- Fix 500 error in dashboard when saving offer directly from form step 1.

- Add created date filtering to Voucher child code export view.

- Fix performance issue in VoucherStatsView when a code had a large number of child codes.

- Revert change to `CREATE OR REPLACE TRIGGER` from `r5.1.1` since this syntax is only supported in PostgreSQL 14.

- Reduce opportunities for deadlocks while updating PostgresSQL views and triggers.

- Add new `max_discount` field to Benefit models, to allow capping the total discount granted by a benefit within a single application. Primary intended use case is for capping the discount granted by a compound benefits, when its child benefits could, in some product combinations, exceed the desired discount.
- Convert README from reStructuredText to Markdown.
- Fix performance issues related to vouchers with large numbers if children.