Migratation unit testing

[Allcharles]

Implement migration unit testing so that changes to the database can be better monitored before deployment to the production server. Currently this is blocked until rails is updated to a version compatible with ActiveRecord::MigrationContext

Relevant links:

• https://docs.gitlab.com/ee/development/testing_guide/testing_migrations_guide.html
• https://gitlab.com/gitlab-org/gitlab/-/blob/f81fa6ab1dd788b70ef44b85aaba1f31ffafae7d/spec/support/helpers/migrations_helpers.rb