Add data migration to associate modern language child subjects

[JamieCleare2525]

Context

• Changes to update Subjects, related to the "Modern Languages", to associated them with the parent subject "Modern Languages"

Changes proposed in this pull request

• PublishTeacherTraining::Subject::Import considers ModernLanguagesSubject its own subject area.
• PublishTeacherTraining::Subject::Import assigns subjects within the ModernLanguagesSubject subject area with the parent subject "Modern Languages"
• Added Data Migration to backfill database with child subject associations for "Modern Languages" associated subjects.

Guidance to review

• run bundle exec rake data:migrate

• This will backfill and language subjects with the parent subject "Modern Languages"

• in the rails console run: Subject.find_by(name: "Modern Languages").child_subjects

• This should return [French, German...]

• run bundle exec rake db:drop db:create db:setup

• in the rails console run: Subject.find_by(name: "Modern Languages").child_subjects

• This should return [French, German...]

in the rails console run:

# Remove all parent subjects
Subject.update_all(parent_subject_id: nil)

# Run the subject importer
PublishTeacherTraining::Subject::Import.call

Subject.find_by(name: "Modern Languages").child_subjects
# This should return [French, German...]

Link to Trello card

https://trello.com/c/BQwACPHr/372-update-subjects-to-use-parent-child-associations

[Nitemaeric]

Can we squash/remove the commits that negate each other? I don't see a reason to add them into the git history.

[ollietreend]

Looks good to me 👍🏻