Support Django 2.2

[willbarton]

This PR makes a few modifications to support Django 2.2:

• The undocumented request._post_parse_error, used in our logging alert handler, was removed in Django 2.1. This PR simply removes the check for it, as we have a generic Exception handler for formatting post data.
• None is no longer encoded in post data by RequestFactory, and values that would be None should. instead be omitted. This PR makes a modification to the feedback tests where referrer and is_helpful were being passed as None if no other value was given.
• schema_json must have a JSON-parsable value. In the jobmanager tests we perform a post to the Wagtail admin and schema_json was not defined. This PR defines it with a blank string.
• Django 2.1+ enforces the use of positional arguments when calling management commands with call_command. In the feedback admin export, we use call_command to call the export_feedback management command, and were previously providing pages as a keyword argument. This PR explodes pages as positional arguments instead.

There is one final category of test failures we're currently seeing, with is_safe_url. @cwdavies has addressed these in #5680. unittest-future will fail here until #5680 is merged and this PR is rebased.

Testing

1.tox -e unittest-future

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the CFPB development guidelines
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Visually tested in supported browsers and devices (see checklist below 👇)
• Project documentation has been updated
• Reviewers requested with the Reviewers tool ➡️

[cwdavies]

All these changes look really good to me.
The changes you made were necessary to upgrade to Django 2.2.