With details of new prices/availability structure.

Add documentation and hide visibility from UI

We currently mention it in internals/sandbox.rst. I've added a note to
the dashboard app (ref/apps/dashboard.rst), the sandbox/urls.py and
expanded on the feature explanation on index.rst.

Fixes #816

They were missing the autodoc documentation for views (abstract) and
abstract models.
Also added two docstrings to the views in the promotions app to have
them showing in the autodoc.

Some of them are rather basic, but it's a start.

- 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

They were getting quite long. They've been split up in several
subsections. Some of them are borrowing heavily from the Django docs.

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.