Add `dry_run` arg to `ryerson_migration` rake task

[camdendotlol]

This PR adds a new dry_run argument that will puts the new user objects to the console instead of saving them.

The arg needs to be called like this: bundle exec rake ryerson_migration[dry_run]

You have to use Bash, rather than zsh, because zsh can't handle brackets.

I have no idea why CLI arguments in Rake tasks are so annoying.

In addition, I've tested the script locally with production data and confirmed that it works correctly.