Use `::find_or_create_by!` in seeds.rb (#1082)
Use `::find_or_create_by!` in seeds.rb (#1082)

Running `rails db:seed` produces an error like so if you run it more
than once:
```
ActiveRecord::RecordInvalid: Validation failed: Name Manufacturer
already exists
<snip>/diaper/db/seeds.rb:190:in `<top (required)>''`
```

This change follows the pattern seen elsewhere in this file to use
`::find_or_create_by!` instead of just `::create`
rubyforgood/diaper,rubyforgood/diaper,rubyforgood/diaper,rubyforgood/diaper