Use seed_fu constraints

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We can improve the way we generate seeds in GitLab development environment, by rewriting our seeds to use Constraints: https://github.com/mbleigh/seed-fu#constraints

This allows seed_fu to be aware of what it has created (using something like database IDs), which allows it to know what it is missing (to skip what already exists) or update existing records to conform with the seed (when we need to change just a tiny example).