Refactor handling of allow_possible_spam flag

What does this MR do and why?

See details on #214739 (closed)

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

See instructions on how to test spam/CAPTCHA here: https://gitlab.com/groups/gitlab-org/-/epics/5527#testing-notes

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.