Add missing migration for Django 2 on_delete behavior

Our Django 2 upgrade introduced an inconsistency in the model state that this migration is rectifying.

Checklist

[x] Changes are limited to a single goal (no scope creep)
[x] Code can be automatically merged (no conflicts)
[x] Code follows the standards laid out in the front end playbook
[x] Passes all existing automated tests
[x] New functions include new tests
[x] New functions are documented (with a description, list of inputs, and expected output)
[x] Placeholder code is flagged
[x] Visually tested in supported browsers and devices
[x] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)