* Adds `/group/:group_id/-/security/vulnerable_projects` which returns a
    list of the group's projects with vulnerabilities
* Adds Vulnerabilities::Occurrence.batch_count_by_project_and_severity
    which batch loads vulnerability counts grouped by severity and by
    project
* Adds Group#vulnerable_projects which returns a list of the group's
    vulnerable projects
* Adds VulnerableProjectSerializer and VulnerableProjectEntity, which
    exposes a vulnerability count method for each severity
* Adds VulnerableProjectPresenter which creates the vulnerability count
    method exposed in VulnerableProjectEntity

https://gitlab.com/gitlab-org/gitlab-ee/issues/11190