Add gitlab-ce migration to fix invalid pool repository SQL rows

Follow-up to #1625 (closed)

source_project_id is NULL in SQL on all rows in pool_repositories as of GitLab 11.10.

In GitLab 11.11 and up, this is fixed for new rows (#1625 (closed)). But people who enabled the object_pools feature flag before then will have invalid rows, and we should write a migration that tries to fix them.