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

Upon implementing previous commit, my changes weren't picked up because
Oscar served the SimpleAddToBasketForm. If a user wants to implement
changes in both forms (which is more likely than it is not), the
previous structure meant they had to be duplicated or mixed in. Now, for
most changes, SimpleAddToBasketForm can be ignored.

The templatetag was actually never used in Oscar. See
`django-rangepaginator` for an alternative.

Deprecated in 1.4

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

instead of South migrations

This allows for customizing product attributes by using the common
get_class() method.

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.

https://github.com/django-oscar/django-oscar/pull/2019

Since there is no way to override SlugField and AutoSlugField in existing Oscar models, it was decided to pass `allow_unicode` parameter if this setting enable. This allows to achieve backwards compatibility with Django versions under 1.9, when this parameter was not supported yet. If parameter specified in the field expliticly - we do not override it, which allow to control slug fields individually in a normal way. For the reasons, explained above, standard SlugField was replaced with a custom one with the single slight change - control over `allow_unicode` parameter by the mentioned setting.