Migration to add unique index to external_services
Created by: miveronese
This PR adds a unique index on external_service.kind, external_service.namespace_org_id so we can enforce the limit also in the DB level. The API already has a check to avoid more than one external service of type Gitlab or Gitthub. For more context, see Cloud-150 and also this old Draft PR.
Test plan
Tested the queries in a local instance and all worked as expected.