Enforce all foreign keys being indexed on Geo database schema

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22694 we are enforcing all foreign keys are indexed, to comply with https://docs.gitlab.com/ee/development/migration_style_guide.html#adding-foreign-key-constraints

But that MR only applies to db/schema.rb. For ~Geo we should ensure the same on ee/db/geo/schema.rb.