Add gitlab-ce migration to fix invalid pool repository SQL rows
Follow-up to #1625 (closed)
NULLin SQL on all rows in
pool_repositoriesas 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.