Submit operating company for approval

Environment-aware submit behavior:

• sandbox: submit directly and move to
  AWAITING_APPROVAL. Resubmitting an Opco already in AWAITING_APPROVAL
  (with a provider id already assigned) is a no-op that just re-queues a
  status poll, rather than erroring.
• production: submit for superadmin review and move to
  PENDING_SUPERADMIN_REVIEW. Resubmitting an Opco already in
  PENDING_SUPERADMIN_REVIEW is a no-op that returns the Opco unchanged.

Only DRAFT Opcos (or the idempotent resubmit cases above) can be
submitted — any other status returns 400. Returns 404 if the Opco does
not exist or belongs to a different client, and (sandbox only) 502 if the
payment network accepted the submission but its response did not include
a Sub-TPP id.