detox is unmaintained and tox has a parallel mode

It is installed separately in transifex.sh for travis, and not required for developing on Oscar.

- Remove outdated material relating to old versions of Django
- State that Oscar assumes that the email field is unique
- Update a number of links to Django documentation.
- Minor fixes to other bits of documentation.

As per [their blog post of the 27th April](https://blog.readthedocs.com/securing-subdomains/) ‘Securing subdomains’:

> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.

Test Plan: Manually visited all the links I’ve modified.

django-compressor used to trigger processing of the less files during
development. But with the browser-based compiler, an easy replacement is
available. This even lowers the barrier to altering styles, because
getting npm up and running isn't required.

Offline compilation isn't impacted.

Using the most current less version seems to not cause any trouble, so I
removed the version requirement.

Vagrant support was removed from release 1.0. This commit removes mentions of it from the docs.

Closes #1559.

This reflects the change in the repo URL.

I much prefer just using npm these days.

The Django admin often enough does not work or not as expected; that
isn't communicated clearly enough. This commit hopefully clarifies
things a bit.

In our instructions, it wasn't entirely clear that virtualenvwrapper is
required for setup.

Fixes #1088

nose-django-querycount gives individual and total query counts. This is
helpful to double-check changes that claim to affect query count
(e.g. #1056).
Total query count gives a quick overview, and diff'ing the output for
two different branches works well.

# Last 5 commit messages
# ----------------------
# 30eead5 Remove custom user app from sandbox [Maik Hoepfel]
# ee3ee02 Drop create_migration.py helper [Maik Hoepfel]
# a77e78c Don't show country field if only one is available [David Winterbottom]
# b5790aa Handle products without a category in GA subission [David Winterbottom]
# 39e0932 Merge pull request #782 from tangentlabs/feature/add_eway_extension_to_docs [David Winterbottom]

Faster is better!

Only search_default needed correcting.

When you use the notation with slashes, use full path to file.
With dots, enter module name. Commands run with the current test set.

Fixes #709

Test suite time has halved.

They really don't belong in /contributing.

Building the sandbox was explained in multiple locations, and in
slightly different ways. The sandbox now has a separate document.

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.

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