Introduce build minutes (again)
This brings back a change introduced by: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/965, but then reverted.
Right now:
- limits are stored as minutes,
- accounting is done in seconds.
This brings back a change introduced by: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/965, but then reverted.
Right now: