Fix invitation emails so that they display the system name and correct

Created by: jogwen

URL (including domain) (kanban card). Issue was that template for invitation email contains references to 'domain' not 'site' - bug introduced at https://github.com/aptivate/django-organizations/commit/31f8af7fc943443973377a439b52dd9b3e930ea1 and then fixed for existing users but not brand new users at https://github.com/aptivate/django-organizations/commit/b24077c1a0f6fc0cf1c599817cdf699456c2a891.