Improve child product handling in dashboard

As part of #675, this PR improves upon the editing flow for child products. A picture speaks a thousand words.

![image](https://cloud.githubusercontent.com/assets/509427/3780512/236e47d6-198d-11e4-94e9-b2e265a79dbd.png)
![image](https://cloud.githubusercontent.com/assets/509427/3780715/f49dadaa-198e-11e4-8b00-454fd744c1a7.png)
![image](https://cloud.githubusercontent.com/assets/509427/3780529/560aa9aa-198d-11e4-8f6d-0067c9fb21b0.png)
![image](https://cloud.githubusercontent.com/assets/509427/3780550/81318108-198d-11e4-8bc2-8f99a1d0b672.png)
![image](https://cloud.githubusercontent.com/assets/509427/3780553/8cf43bd4-198d-11e4-82d0-99dbc2c93fdf.png)
![image](https://cloud.githubusercontent.com/assets/509427/3780583/d9c51e56-198d-11e4-977d-6d3c0fb27c62.png)

It's nothing radical. The ``ProductCreateUpdateView`` now has a third URL route to specify a parent product for creating a child product. When creating or updating a child product, the template considerably limits the displayed information. I explicitly did not do more complex hiding, because I feel everything else would impact customisability too much. Furthermore, an entry to the product list action drop down and to the product editing buttons has been added if appropriate.

I'm pretty happy with this. It'd be nice if @codeinthehole would find the time to review. Maybe @mbertheau can also have a look to see if I break anything for their rather customised product dashboard.