Add two important partial index in postgres

- `prs_to_update` for `Repository.fetch_updated_prs`
- `closed_without_closed_by` for
`Repository.fetch_closed_issues_without_closed_by`

Fixed #153
Use field in queryset instead of fk

`closed_by__isnull=True` => `closed_by_id__isnull=True`

Done during #153