Add Rails/SafeNavigationWithBlank cop

[gyfis]

This PR adds a new Rails/SafeNavigatinoWithBlank cop.

It's a clone of the base Ruby cop Lint/SafeNavigationWithEmpty updated for .blank? in Rails, which has the same issue.

It's up for discussion if the cop should cover all usages of safe operator &. and .blank?, and not be limited only to conditionals. This could also be an option of the cop.

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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.

[gyfis]

@koic Thanks a lot for the review!

I fixed the obvious remarks and also took a shot at implementing autocorrect for the cop - during that, I also fixed the pattern matcher a bit (I think it was wrong before).

Let me know if I can do more on this PR!

[koic]

Thanks!