Fix Postgres issue not dropping null constraint

[bjester]

Background

In #143 we revised an existing migration which added new columns to make the fields nullable. In addition, we created another migration that also drops the null constraint if the previous migration had already ran. This appears to work fine for SQLite but not for PostgreSQL due to something internal to Django (1.11 at least which is used in Kolibri).

Summary

• Resolves issues with modified migrations that don't apply correctly with Postgres

TODO

• Have tests been written for the new code?
• Has documentation been written/updated?
• New dependencies (if any) added to requirements file

Reviewer guidance

• See postgres test failures here before changing the migrate_to of the test to the new migration

Issues addressed

learningequality/kolibri#9139

Documentation

If the PR has documentation, link the file here (either .rst in your repo or if built on Read The Docs)

[rtibbles]

Code changes make sense, test covers it.