Add cascading namespace setting database migration helper

What does this MR do?

Closes #327033 (closed)

Add a migration helper to simplify adding a new cascading setting. The helper will create the four required columns, two each in namespace_settings and application_settings.

Developer documentation will be updated along with !58917 (merged).

Usage:

class AddDelayedProjectRemovalCascadingSetting < ActiveRecord::Migration[6.0]
  include Gitlab::Database::MigrationHelpers

  def change
    add_cascading_namespace_setting :delayed_project_removal, :boolean, default: false, null: false
  end
end

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is not a user-facing change.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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