This commit adds JSON endpoints for fetching vulnerability findings
for the instance security dashboard.

* Adds Security::VulnerabilityFindingsController, which includes the
    ProjectCollectionVulnerabilityFindingsActions
* Adds SecurityHelper
* Adds ApplicationInstance, a PORO model that includes Vulnerable and
    represents the entire GitLab instance
* Adds vulnerability findings routes
* Adapts the vulnerability history cache for use with the instance
    dashboard

https://gitlab.com/gitlab-org/gitlab/issues/33899