Update ConditionsWidget to support Django 1.6 and higher.
Review Request #8681 — Created Jan. 30, 2017 and submitted
ConditionsWidget was previously passing in choices to the render method for the choice widgets, which is no longer supported on modern versions of Django. To provide the same behavior, we need to temporarily update the widget's list of choices to be those that we want to render on the page. This introduces a new internal context manager and some small logic changes to temporarily append new choices to the widgets for rendering purposes, reverting them after render is complete. This should be safe, as we should be working off of a local copy of the widget and its state and not the global version.
Unit tests pass on Django 1.6, 1.7, 1.8, 1.9, and 1.10.
Tested manually against 1.6 that the widgets were still rendering and
performing validation correctly.