Related to #823

Dependencies installed via setup.py stay in their eggs and their
templates won't be found by Django. This caused a
TemplateNotFoundError when users were trying to use
Django's admin to set categories.
Using the admin is unsupported, but this still prevents a similar error
from happening when we use other dependencies that ship templates. It
also shouldn't slow down production use.

Fixes #761

All credit for those findings goes to Dawid Lorenz.

Fixes #709

* Better wordings for the individual steps
* Mentioned necessary copying of migrations directory
* Fixed wrong prerequisites in examples
* Glossary typo fixed

- 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

Fixes #574

- Made instructions on using a custom template more expliciy.  Should be
  better for new users.
- Fixes a few typos and mistakes in the code examples.

This commit is squash of pull request #640.

It's a great idea, but confusing when empty. Replaced by links to
how_to_customise_models which contains some explanations.

Attempt at making it clearer what needs to be done for what kind of
customisation. Created a new topic that lines out the prepatory
steps that need to be taken, and moved duplicate information from the
how-tos into the new topic.

The old howto included out-dated info on how to translate Oscar. That
has been replaced by a link to the Transifex project.
It was the only howto left under the Contributing headline, which really
shouldn't be there (as there's an entirely different section in the docs
for contributing). As it is more of a topic anyway, and will hopefully
soon include documentation on translation style etc., it was turned into
a topic.

Was posted on the mailing list.

my_model_customisations_arent_picked_up now lives in a subsection of the
recipe how_to_customise_models.

The old conventions.rst is now merged into the new coding-style.rst.

The old search.rst has been dropped as it seems outdated.

/howto/index.rst was pointing at the wrong file. Renamed the file to
maintain naming consistency.

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.