1👍
As the error says, the behavior of the template url tag changed in Django 1.5. It used to take only plain text as the view name, so it didn’t have to be quoted. It’s useful to be able to pass in view names as context variables, so the syntax changed. The docs describing this change say:
In Django 1.5, the behavior of the url template tag will change, with the first argument being made into a context variable, rather than being a special case unquoted constant. This will allow the url tag to use a context variable as the value of the URL name to be reversed.
https://docs.djangoproject.com/en/1.4/ref/templates/builtins/#url
You can still just use a string constant if you quote it appropriately, though – your email template should look like this:
http://{{ site.domain }}{% url "registration_activate" activation_key %}