Add missing app_labels for offer app

Closes #1485.
Add missing app config trigger for offer app
Specify fields explicitly

This will be required by Django 1.8, and Django 1.7 complains with
DeprecationWarnings.
Correct comment

The functionality to browse all products has been explicitly moved to
CatalogueView. ProductCategoryView now complains if it didn't get a category to
display products from.