Also remove unused migrations test for MySQL.

Use the same option in the virtualenv command as the mkvirtualenv command uses.
apt-get is nowadays shortened to apt.

detox is unmaintained and tox has a parallel mode

The ability to include(admin.site.urls) was removed in Django 2.

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- Remove outdated material relating to old versions of Django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.

Load i18n context processor from django.template instead of django.core as fix for Django 1.10

Since there is no demo or us site anymore we can simplify the structure
somewhat by moving the sandbox one directory up

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.

Conflicts:
	docs/source/internals/getting_started.rst

Cherry-picked from #1934, because that PR was onto releases/1.1 and I
merged it to there by accident.

The demo site was not kept up to date with the recent changes in Oscar
due to time contraints. It should be moved to a separate demo repository
in the future.

This might be resurrected in a separate repository in the future.

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.