Refactor allow possible spam feature flag into an application setting

What does this MR do and why?

This MR refactors the allow_possible_spam feature flag into an application setting. The current value of the feature flag is false and the default value of this new column is false too.

This setting allows to:

• have Akismet enabled for monitoring purposes
• have reCAPTCHA disabled and accept all changes

The logic is as follows when creating or updating an issue or a snippet:

• Akismet flags as spam?
  • yes:
    • allow_possible_spam = true: allow and log (OVERRIDE_VIA_ALLOW_POSSIBLE_SPAM)
    • allow_possible_spam = false:
      • captcha_enabled = true: conditional allow with CAPTCHA and log (CONDITIONAL_ALLOW)
      • captcha_enabled = false: do not allow and log (DISALLOW)
  • no: allow and do not log (ALLOW)

(the constants are from Spam::SpamConstants)

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/33421

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