But make sure we take into existing entries into account.

This enables us to add new entries to the seeder, so they will be
automatically created in existing deployments on horizon restart (or
when triggering the seeder manually).

This is better than using migrations because:

• We avoid issues from seeder and migrations both trying to create the same entries.
• We don't mix database layout and content changes. Content changes as migration can be problematic if the state that the migration expects wasn't actually created by the migration.

Seeders at least must take into account being rerun, where it makes
sense a seeder can also update already created entries with new
configuration values.