Add support for the Kea pool-ids in the backend and in the UI

The #1208 (closed) extends the subnet form with the ability to edit the pools. However, it lacks the ability to specify the pool-ids for the pools. The pool-id is a relatively new concept in Kea and neither Stork database nor the API is ready to store the pool-id. This ticket should extend the database, the API and the UI to support the pool-id management. For older Kea versions, it should be possible to omit pool ids.