Add unique index to delivery_configurations and add DeliveryConfigurations for seeds

[stephencdaly]

What problem does this pull request solve?

Trello card: https://trello.com/c/WMnjLlJV/

There can only be one delivery configuration per form with the same delivery_method and delivery_schedule. Add a unique index to enforce this at the database level.

Also add DeliveryConfigurations for the seed forms.

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[github-actions]

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