[RUN ALL RSPEC] Load Rails 5.1 configuration defaults

What does this MR do?

Part of #332288 (closed)

From https://guides.rubyonrails.org/configuring.html#results-of-config-load-defaults, calling load_defaults 5.1 sets the following values:

config.assets.unknown_asset_fallback: false
config.action_view.form_with_generates_remote_forms: true

config.action_controller.per_form_csrf_tokens: true
config.action_controller.forgery_protection_origin_check: true
ActiveSupport.to_time_preserves_timezone: true
config.active_record.belongs_to_required_by_default: true
config.ssl_options: { hsts: { subdomains: true } }

Some of these we set back to the previous values because they're not compatible with our code.

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