Add new Rails/ActiveSupportOnLoad cop.

[bdewater]

See rubocop/rails-style-guide#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.

[bdewater]

Not sure what's going on with the test failures - they don't seem related to the changes and locally on the master branch seeing the same errors.

[bdewater]

Thank you for your suggestions @koic 🙇‍♂️ it led me to a Rails page that showed many more load hooks, I added these as well.

[koic]

@bdewater Thanks!