re-add flip cards

...ations/0252_alter_districtarticlepage_content_and_more.py

...ons/migrations/0047_alter_electionsarticlepage_content.py

main/migrations/0106_alter_mainarticlepage_content.py

shared/blocks/__init__.py

shared/blocks/parents/struct.py

...grations/0098_alter_uniwebarticlepage_content_and_more.py