Add rubocop-rspec_rails gem and fix RSpec lint violations

[giverm]

Changes

• Add rubocop-rspec_rails gem and configure in .rubocop.yml
• Enable RSpecRails cops for HTTP status and validation matchers
• Update HTTP status codes from :unprocessable_entity to :unprocessable_content
• Replace numeric status checks (422) with semantic matcher have_http_status
• Replace be_invalid with not_to be_valid for better RSpec idioms

Preview environment for reporting-app

♻️ Environment destroyed ♻️

[giverm]

For this work, I initially set out to add a RuboCop Cop to enforce the use of :unprocessable_content over :unprocessable_entity since the latter is now deprecated. That already existed in the rubocop-rspec_rails plugin, so I figured I would just add that for improved consistency in our specs. Let me know if people are ok with this update.

[doshitan]

Seems fine to me if we only enable the "safe"/default cops and any of the non-default ones we want explicitly. And this can be added to https://github.com/navapbc/template-application-rails.

[giverm]

Thanks for the review, @doshitan! I've move part of this work over to the rails template navapbc/template-application-rails#137. I've also updated to disable the RSpecRails/InferredSpecType cop and an explicitly enable a few others rather than keeping NewCops: enable