Make Rails/EnvironmentComparison aware of Rails.env.to_sym

[lovro-bikic]

Adds support for Rails.env.to_sym to Rails/EnvironmentComparison cop. Code like:

Rails.env.to_sym == :production

will be registered as an offense and autocorrected to:

Rails.env.production?

Rails.env.to_sym is used relatively commonly, and there are some examples of it being used to check the current env, so I think this is a worthwhile addition to the cop.

If this PR is accepted, Rails/UnknownEnv should also be updated to support Rails.env.to_sym.

Lastly, I took the opportunity to refactor the cop a bit. Let me know if it's too much for this PR and I'll revert the changes.

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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.