Two links needed to be corrected.

- Added stubs for both apps
- Switched to using :automodule: for abstract models instead of
  individual :autoclass:
- Added instructions on creating a Partner and ProductClass instance
  when setting up a new Oscar instance

To set a base for upcoming docs changes, the docs directory structure
has been streamlined to follow a simplified version of Django's docs.

The old _draft folder has been removed. Both files from there and the
current docs that don't fit nicely into the new structure are in _old/
for the moment.

This allows partners to be identified without using a name that might
change or be translated.

Unfortunately, this involves a few tricky migrations to move data around
in the correct order.

Fixes #434 but reverts work done for #354