Add new Rails/RedundantForeignKey cop

[eugeneius]

The foreign key for an association can be overridden with the :foreign_key option. This cop detects when the specified value is the same as the default.

Aside from being unnecessary, setting the foreign key prevents the inverse association from being inferred automatically, which can lead to unnecessary queries.

The default foreign key logic implemented here matches what Rails does internally:

https://github.com/rails/rails/blob/95af87f0a4b6274a43853ed68b17e17c21f804f3/activerecord/lib/active_record/reflection.rb#L659-L667

I don't think this warrants an entry in the Rails Style Guide on its own, but a more general section about ensuring associations have inverses that mentions foreign_key as a gotcha might be useful.

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Before submitting the PR make sure the following are checked:

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

Thanks!