DRY vulnerability feedback permissions checks

The following discussion from !9700 (merged) should be addressed:

Proposal

Since https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9700 we're checking abilities for creating vulnerabilities in several places:

  • ee/app/helpers/ee/projects_helper.rb
  • ee/app/serializers/vulnerabilities/occurrence_entity.rb
  • ee/app/helpers/ee/projects_helper.rb

We should try to remove that duplication.

Edited by Olivier Gonzalez