Merged ProductUpdateView and ProductCreateView

There's a lot of code duplication between the two. This is an attempt of
merging the two.

* The additional formsets are now easily overriden.
* is_valid logic is more concise
* is_stockrecord_submitted is now more robust
* Fix: Partner and Partner SKU were optional. They're required, only the
  entire form is optional.
* Fix: StockRecordForm errors are now displayed
* StockRecord is now deleted when clearing all fields