Make product classes manageable within the dashboard closes #1094

Product classes can now be created, updated and deleted via the catalog drop-down in the dashboard. Some thoughts:

I chose 'Product class' instead of 'Product type' as that is how it appears in the model and want to keep things consistent, though I believe 'type' would make more sense to a user.

When classes are deleted, all products that belong to that class are deleted. Is this desirable behavior?

This is my first contribution through Github and in a Django project so feedback is appreciated.