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.