Support Wagtail up to 2.5

This PR bumps support for versions of Wagtail up to 2.5, and makes unittest-future test against Django 2.2 and Wagtail 2.5.

Checklist

[ ] 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 front end playbook
[ ] Passes all existing automated tests
[ ] New functions include new tests
[ ] New functions are documented (with a description, list of inputs, and expected output)
[ ] Placeholder code is flagged
[ ] Visually tested in supported browsers and devices
[ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)