Update defaults in dummy application

[waiting-for-dev]

Here it's a description of the reason for each change. Keep in mind that
CI test against Rails 5.2 as the minimal version:

• serve_static_assets: Removed it. It's not available since Rails
  5.0.

• public_file_server.enabled: Set explicitly to true. Even if it was
  already true, a real Rails application will default it to false in
  the production environment unless a env variable
  RAILS_SERVE_STATIC_FILES is
  present. This way it's less surprising.

• whiny_nils: Removed. It's not available since Rails
  4.1.

• action_controller.allow_forgery_protection: Set to true to mimic
  production.

• action_controller.default_protect_from_forgery: Set to true to mimic
  production.

• action_controller.perform_caching: Set to true to mimic production.

• active_support.deprecation: Remove duplicated entry.

• action_mailer.delivery_job: Removed because load_defaults take care of
  it.

• storage_path: Removed. It wasn't a Rails setting, but used as a
  convenience. It was confusing.

• action_controller.include_all_helpers: Removed as it's already the
  default.

• log_level: Added. It already defaulted to :debug, but a real Rails app
  will default to :info in production. To make things less surprising
  let's be explicit.

• action_mailer.perform_caching: Added to mimic production.

• i18n.fallbacks: Added to mimic production.

• active_record.dump_schema_after_migration: Added to mimic production and
  for better performance.

• assets: No need to check whether it's a method on config.

See #4035

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)
• I have attached screenshots to this PR for visual changes (if needed)

[waiting-for-dev]

Hmm. Some tests failing on CI but not on my local machine. I'll try to figure it out.

[waiting-for-dev]

I think it'll be green once #4048 is merged

[kennyadsl]

Thanks, @waiting-for-dev.

[kennyadsl]

@waiting-for-dev did we remove the grouping suggested by Jared with the last commit?

[waiting-for-dev]

@waiting-for-dev did we remove the grouping suggested by Jared with the last commit?

Thanks for catching it @kennyadsl . As I directly committed the suggestion but didn't update my local repo, I override when forced the push 🙈 Sorry for that 😞

Just fixed it.

[kennyadsl]

No problem, we are here to review!