https://gitlab.com/gitlab-org/gitlab/merge_requests/17316 introduced
new user_id and created_at columns for `Versions`
https://gitlab.com/gitlab-org/gitlab/merge_requests/18506 renamed this
column to author_id.

This MR has the app capture a `author_id` when new versions are created.

It also contains a post-migration to backfill author and created_at data
for `Version`s by sourcing from `Commit`s.

https://gitlab.com/gitlab-org/gitlab/issues/32081
https://gitlab.com/gitlab-org/gitlab/issues/13426