[Fix #546] Exclude app/models by default for Rails/ContentTag

[koic]

Fixes #546

This PR excludes app/models by default for Rails/ContentTag. Because it prevents false positives for tag calls to has_one: tag. No helpers are used in normal models.

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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.
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• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.