Make product classes manageable within the dashboard closes #1094
Disallow deletion of product classes with products. ‘Class’ changed to ‘Type’ in UI.
Fix whitespace and line length
Rename 'product class' to 'product type' when facing customer

It's easier understood by the everyday person than product class.

Tried to have consistent capitalisation. "Product Type" when used by
itself, "product type" when used in a sentence, apart from when used
in the beginning of a sentence ("Product type...").
Don't show slug in product class list

It's not really crucial to know.
Merge branch 'elliotthill-master'

Conflicts:
	oscar/apps/dashboard/catalogue/views.py

Fixes #1094
Fixes #1174