Parallelize development fixtures for 13_comments

What does this MR do and why?

Speed up 13_comments seeder by up to 80%

This parallelizes the 13_comments seeder, reducing the duration by around 80% on macOS (~152 to ~32).

As identified previously, this fixture seeder is a major contributor to the duration of all seeding. While we have added a --fast option to reset a seeded GDK database in less than a minute, seeding from source is still the default operation and also runs in CI.

References

See Parallelize fixture seeding (#480716 - closed).

How to set up and validate locally

1. Run bundle exec rake db:seed_fu FILTER=13_comments
2. See the duration
3. Run the same command on master
4. Compare the two values

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