Move `ee/app/helpers/policy_helper.rb` to `ee/app/helpers/projects/threat_monitoring_helper.rb`

The following discussion from !66416 (merged) should be addressed:

• @ifarkas started a discussion: (+2 comments)

  Why do we split the data between the 2 helper classes (Projects::Security::PoliciesHelper and PolicyHelper)?

Summary

• ee/app/helpers/policy_helper.rb, which was given the wrong name/location that does not follow the convention, should be moved to ee/app/helpers/projects/threat_monitoring_helper.rb
• The rest is covered in #341066 (closed)

Risks

The policy_helper.rb file contains the alert details helper method, which needs to continue to work after the move

Involved components

• https://gitlab.com/gitlab-org/gitlab/blob/master/ee/app/helpers/policy_helper.rb