We use Django's template language for allowing people to render custom webhook

content, but we deliberately limit the tags and filters that people are allowed

to use. Previously we just instantiated the Parser and told the resulting

nodelist to render, but in Django 1.8 there's some code that relies on having a

Template object for state. This change creates a Template and then replaces

its nodelist with our parsed version.

Ran unit tests with Django 1.6 and 1.8.