v5.0: Improve order numbers for database orders

[duncanmcclean]

This pull request makes some changes to how order numbers are handled when using Database Orders.

Previously, when storing orders in the database, Simple Commerce has used the id column as order numbers. This meant order numbers started in the single digits and it wasn't possible to set a minimum order number (eg. 1000) like you can with entry orders.

With this PR, I've introduced a separate order_number column to store order numbers so they can be different.

A couple of notes

• Order numbers for orders will be generated when creating orders
• During the upgrade process, a migration will be published & existing orders will be updated.

[github-actions[bot]]

Released as part of v5.0.0.