Add Rails/ActionControllerFlashBeforeRender cop

[americodls]

This PR suggests a new Cop for detecting flash assignment before render a page (the correct approach would be to assign a message to flash.now (check on rails guides) .

This is a common mistake in Ruby on Rails, and it is easy to fall into this trap the flash will be rendered on the page. But the flash message still persisted and will show up on the following redirect.

This is an example of that issue: rubygems/rubygems.org#3149

Solved here: rubygems/rubygems.org#3198

This is a very simple version but helped me to find more 6 issues on that project.
I hope I can receive some suggestions on how to improve this solution.

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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.

[simi]

Is there any way to scope it only to Rails controllers 🤔?

[americodls]

I added some changes to make the cop more specific and tidied up the tests.
Now the cop will check only instance methods and blocks passed to macros in a rails controller class.

[koic]

Thanks!