Use the right template context processors on Django 1.11.
Review Request #10526 — Created April 11, 2019 and submitted
Django moved their core template context processors into a new module
in 1.8. This updates our settings to pull in the right context
processors for both Django 1.6 and 1.11.
Unit tests pass on Django 1.6 and 1.11 (with other in-progress changes).