setup django-test-migrations

[fyliu]

Overview

We need a solution to test data migrations automatically. The tool django-test-migrations offers this possibility but we need to do a trial before we adopt it.

Action Items

• Set up django-test-migrations
• Write tests for existing data migrations (both forward and backward)
• Check that there's full test coverage of the migration files
• Create an mkdocs page describing how to use django-test-migrations

Resources/Instructions

• https://github.com/wemake-services/django-test-migrations
• https://stackoverflow.com/questions/44003620/how-do-i-run-tests-against-a-django-data-migration/59016744#59016744
• Mkdocs documentation to be updated