Synchronize advisories database
When Gemnasium will be part of the omnibus package (gitlab-org/gitlab-ee#5475), the advisory database will be isolated from our main DB. We need a way to synchronize it locally.
Gemnasium Enterprise was using a dedicated syncer to fetch relevant updates, but it was sending the package names (no versions) to determine the data to sync. This is pretty much what we try to avoid with this issue.
Warning: if we simply provide and expose all our advisory DB, we don't control its distribution anymore.