Batch insert for GitLab import

Currently, GitLab import performs single-record inserts in https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/import_export/project_tree_restorer.rb#L100. In order to improve performance, we should look into performing batch inserts.

Batch inserts typically improve performance drastically. The downside is that we are not going to be able to trigger any model callbacks.