Add new Rails/Pick cop

[eugeneius]

pick(...) is a shorthand for pluck(...).first.

https://api.rubyonrails.org/classes/ActiveRecord/Calculations.html#method-i-pick

This cop is unsafe because pluck is defined on both ActiveRecord::Relation and Enumerable, whereas pick is only defined on ActiveRecord::Relation in Rails 6.0. This will be addressed in Rails 6.1 via rails/rails#38760, at which point the cop will be safe. I don't know if there's currently a way to apply a different default configuration based on the Rails version, but I'd be happy to investigate that once Rails 6.1 is released.

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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.
• 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!