Uniform paths joining in the code generated by rails new

[vlado]

Summary

Adds uniformity when joining paths (Rails.root.join(...)) in the code generated by rails new command.

rails new currently generates the code with slashes in this places:

https://github.com/rails/rails/blob/6-0-stable/railties/lib/rails/generators/test_unit/generator/templates/generator_test.rb.tt#L7
https://github.com/rails/rails/blob/6-0-stable/railties/lib/rails/generators/rails/app/templates/config/storage.yml.tt#L3
https://github.com/rails/rails/blob/6-0-stable/railties/lib/rails/generators/rails/app/templates/config/storage.yml.tt#L7
https://github.com/rails/rails/blob/6-0-stable/railties/lib/rails/generators/rails/app/templates/config/storage.yml.tt#L21

And using arguments here:

https://github.com/rails/rails/blob/6-0-stable/railties/lib/rails/generators/rails/app/templates/config/environments/development.rb.tt#L17 (this PR changes it to slashes also).

Other Information

Based on my personal experience, having Rubocop running while teaching school kids Ruby/Rails is a huge time saver. No need to point them to same mistakes all the time. We also create new Rails apps a lot and each time config/environments/development.rb:17:6: C: Rails/FilePath: Please use Rails.root.join('path/to') instead. offence is reported. We have rule disabled for a long time and I finally gather up the courage to submit this PR 💪😄.

I understand that this change could be considered cosmetic in nature and not accepted but the change from slashes to arguments https://github.com/rails/rails/pull/31530/files was in fact also cosmetic cause we already knew it does not change anything https://twitter.com/tenderlove/status/842064491936280576 😄

[p8]

This seems like a valid change. I'm also seeing more argument joins in the dummy apps config.

[rails-bot]

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