On Django 1.6, we replace the {% url %} template tag with our own by

adding it to the built-in list of tags through the add_to_builtins()

method. On newer versions, we have a much cleaner way of doing this by

listing the module as a built-in template tag library in settings.
This change makes use of the new setting, and avoids calling

add_to_builtins() on newer releases where it doesn't exist.

Unit tests pass on Django 1.6 and 1.11.
Manually tested the site on both to ensure that {% url %} was

working with Local Sites and non-Local Sites.