Revert "Fix Slack link in readme, and remove link to non-functional sandbox site"
Revert "Fix Slack link in readme, and remove link to non-functional sandbox site"

This reverts commit 9f547c2095c176b0d7e26ea211ccdaf8f9fb24aa.

sasha0/django-oscar,sasha0/django-oscar,django-oscar/django-oscar,sasha0/django-oscar,django-oscar/django-oscar,solarissmoke/django-oscar,sasha0/django-oscar,solarissmoke/django-oscar,solarissmoke/django-oscar,django-oscar/django-oscar,solarissmoke/django-oscar,django-oscar/django-oscar