Improve delivery workflow

For a long discussion, see foodcoops#103 (English) and foodcoop-rostock#39 (German).

Suggestion how to polish the delivery form. The form currently used is very hard to fill because there is no possibility to add/edit StockArticles without leaving the page.

There are quite a lot features included in this pull request. Since this is a main feature in a coop with stock, it should absolutely be worth merging. In my coop, several hours per week are spent filling this form.

I gave my very best to avoid the introduction of bugs. Furthermore, I would be happy to discuss about some aspects
- [ ] check proper escaping
- [ ] Some more javascript functions in global namespace OK? I do not mind.
- [ ] discuss about js.erb (I prefer it over js.haml due to better indentation)
- [ ] allow for external database import?
- [ ] ...