This adds support for validating the cache backend in the General

Settings page on Django 1.11. We no longer have access to get_cache(),

which was present on Django 1.6, so we now duplicate the functionality

by importing the backend manually and then constructing an instance of

the backend.
In order to test this on Django 1.11, a change needed to be made to the

cache field display logic for the form. Modern versions set the

required attribute on required fields, which meant that the form

couldn't be submitted due to the hidden fields for other backends. We

now dynamically set the required field based on the current backend

and the presence of a required CSS class on the label (which is

available on both).

Tested the form on Django 1.6 and 1.11. Checked for both validation

failures and successful validation.