In order to centralize po_number generation, but still allow the UI to suggest a value, we need an API endpoint that returns a generated po_number.
po_number generation is already happening in mod-orders (PostOrdersHelper), so that code can be refactored for use here. The current implementation can be scrapped and replaced with a call to the new GET /orders-storage/po_number endpoint. Later we may add a prefix and/or suffix to the value obtained, but that's beyond the scope of this story.
- GET /orders/po_number is defined (e.g. in RAML/ModuleDescriptor) and implemented (code and schema)
- returns an appliacation/json response with a single field "po_number" (status 200)
- Unit tests