add page unique constraints

[gardenerik]

[krtko1]

need to do something like this but more complicated because countries is ArrayField and we want UniqueConstraint ["slug", "branch", "language"] and country form countries? https://stackoverflow.com/questions/59571895/make-base-field-of-arrayfield-unique-for-a-table-in-django

maybe it can by done through https://docs.djangoproject.com/en/4.0/ref/models/constraints/#condition ? Not sure how