Allow to specify an order schedule for new orders.

Especially when working with several suppliers, it can be a little cumbersome to open all orders. This change allows a foodcoop to specify a schedule for the order end date, so that it is filled in automatically when creating a new order (it can still be changed for the order when needed).

The configuration option ended up in the 'finance' tab. I considered to rename it to 'orders', but as it contains VAT it doesn't fully make sense either. I'm a bit careful to add more and more tabs. For now, I'd be ok with this, until some growth of options brings out a more sensible division between tabs.

Also adds integration tests for opening orders. Yay!