Add new Rails/ActiveSupportOnLoad cop.

[bdewater]

See https://github.com/rubocop/rails-style-guide/pull/323

This cop is extracted from Shopify's internal Rubocop repository. Many thanks to the original authors for their work.

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Before submitting the PR make sure the following are checked:

• [ ] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [ ] Wrote good commit messages.
• [ ] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [ ] Feature branch is up-to-date with master (if not - rebase it).
• [ ] Squashed related commits together.
• [ ] Added tests.
• [ ] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [ ] Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• [ ] If this is a new cop, consider making a corresponding update to the Rails Style Guide.