fixes bug 785271 - Add admin interface for "add new product"

@rhelmer r?

The way I tested this (apart from a bunch integration tests) was to edit `products.py` and `releases.py` so that, in the moment before calling the relevant stored procedures, it instead raises an exception and I print what it was about to call the stored procedures with. I did this because my middleware is hooked up to the stage DB and I didn't want to create some random junk in there. 

Also, on the Admin interface for `/admin/products/` and `/admin/releases/` there's good opportunity to either explain some of the fields or to make some necessary links to the documentation. What do you think?