These migrations do the following:
- Adds a new `issue_id` column to `versions`. This fixes an n+1 problem
when loading versions for an issue in GraphQL as AR can now load from
cache
- Change the unique restraint on versions.sha to be scoped to `issue_id`
as in order to import version data, we need to allow duplicate `sha`
values for versions
- Update all versions with an `issue_id`
https://gitlab.com/gitlab-org/gitlab-ee/issues/11090
