Allow adding a free price to member order

[wvengen]

This is needed for the checkout / receive>ordergroup screen.

New approach

Use supplier with foodcoops-specific articles (see https://github.com/foodcoop-adam/foodsoft/issues/159#issuecomment-137209467). No changes required :)

Old approach

What needs to be done? (See first comment for an explanation.)

• [x] Add articles to bottom selectbox in current_orders/ordergroups screen
• [ ] Figure out a way to put both OrderArticles and Articles in the selectbox
  • either use two selectboxes
  • start using globalids instead of ids
  • some javascript to set either order_article_id or article_id based on choice
• [ ] In order_articles_controller, also create OrderArticle when needed
• [ ] Give non-ordered articles a dimmed colour
• [ ] Merge into foodcoop-adam fork

First implement this upstream, then merge in this fork.

[wvengen]

This could be done by creating a sort of dummy article called "vrije verkoop" with an fc-price of €1. When set to not-available, it will not show up in member's lists when ordering.

What remains, is to be able to choose this article from current_orders/ordergroups. Currently a list is shown with all articles in the current orders. This needs to be expanded to also show articles not in the current orders (restricted to all suppliers taking part in the current orders).

[wvengen]

New idea requiring no changes: creating a "Foodcoop" supplier with a "Vrije verkoop" product. Open the order on the pickup day, and close it right away. Then it will be available.

Con: you can't add articles that weren't ordered this month.