Create a CI job that test new migrations with a lot of meaningful data
As described in https://gitlab.com/gitlab-org/gitlab-ce/issues/32341#note_29903427, we should create a new CI job that:
- Checkout the target branch
- Run migrations
- Populate the DB with a lot of data: gitlab-foss#28149 (moved) / https://gitlab.com/gitlab-org/gitlab-ce/compare/master...28149-improve-seed
- Merge the branch under test
- Run the new migrations
- Report timings of the new migrations