Enforce using symbolic status codes in controllers

Igor Drozdovrequested to merge
id-enforce-symbolic-status-codes-in-app into master

This commit removes Rails/HttpStatus cop out of todo and corrects the existing offenses.

We've agreed on using the symbolic expressions in tests and there's even WIP MR for this: !17705 (merged). Enforcing this rule in controllers is a separate cop and relatively small amount of works, so why not enable it?

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