[BE] Improve provisioning and onboarding of new sales-assisted GitLab.com subscribers
## Summary
This customers-gitlab-com~2492649 issue (for https://gitlab.com/groups/gitlab-org/-/epics/5012) will be used to track refinement work. As a result, new BE issues will be created.
## Design
[See UX issue](https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/2040/)
epic