Add ConstantInRoutes Cop

[skipkayhil]

In applications with large route files, its common to have a desire to extract regexes, constraints, etc. that are used multiple times to prevent duplication. If the value is never going to change, extracting the value to a constant would be a very normal thing to do in typical Ruby classes/modules. However, the config/routes.rb file is reloadable, so a constant defined there will causing a constant redefinition warning when/if the routes are reloaded (typically during development or test, especially if using Spring).

This commit adds a ConstantInRoutes Cop to prevent constants from being defined in config/routes.rb files.

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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.