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
- [x] check proper escaping
- [x] Some more javascript functions in global namespace OK? I do not mind.
- [x] discuss about js.erb (I prefer it over js.haml due to better indentation)
- [x] allow for external database import? (Later)
- [x] Add a hint to copy the stock_article if a price change is desired
- [x] Adjust table sorting style to existing one
- [x] Style bootstrap tooltip in light colors
- [x] Show FC markup in price info
- [x] Separate columns for price and unit
- [x] Allow creation of stock_articles even if there are zero non-stock articles for that supplier
- [x] testing (manually)