Fix flash before render and add custom cop

[americodls]

This PR introduces a custom cop to detect and prevent bugs like this one: #3149

Similar bugs were fixed as well in this PR.

Another PR was sent to rubocop-rails: rubocop/rubocop-rails#759

If that PR is accepted we can update the rubocop-rails version and delete the custom cop from this project.

[codecov]

Codecov Report

Merging #3201 (65388f3) into master (f4e4bf7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3201   +/-   ##
=======================================
  Coverage   98.18%   98.18%           
=======================================
  Files         110      110           
  Lines        3310     3310           
=======================================
  Hits         3250     3250           
  Misses         60       60           

Impacted Files	Coverage Δ
app/controllers/api_keys_controller.rb	98.27% <100.00%> (ø)
app/controllers/multifactor_auths_controller.rb	100.00% <100.00%> (ø)
app/controllers/owners_controller.rb	98.03% <100.00%> (ø)

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[simi]

ℹ️ failing CI seems not related. I'll restart it later. Good job for now 💪.

[americodls]

Hey @simi

Now the new version of rubocop-rails was released with the feature I've implemented based on our previous discussion that detects potential misuse of flash, I think we can close this PR and I can send other PR where:

• rubocop-rails is upgraded to the latest version
• .rubocop_todo.yml is regenerated
• Rails/ActionControllerFlashBeforeRender offenses are fixed

After merging this new PR, all similar bugs to that one will be fixed and we must not see them anymore.

What do you think?

[americodls]

@simi I just created the PR -> #3207