GitHub project importer should update `internal_ids`
This is part of: https://gitlab.com/gitlab-org/gitlab-ce/issues/49446
The GitHub importer should adhere to the internal id scheme. In particular, we need to make sure we track the greatest internal id ever assigned. Since those may come from external systems like GitHub (when we import the project), we need to keep track of those in
A way to track the greatest id ever seen is provided with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20626.