Update to the new "web actions" API endpoint

[l2fprod]

experimental/web has graduated.

Dark Vision should move to the new API.

https://medium.com/openwhisk/whats-new-web-actions-e0ffeef1bea3

Note that if you remove "experimental" today from the STT_CALLBACK_URL, it will lead to an error like:

{
  "error": "Response is not valid 'message/http'.",
  "code": 3104139
}

because the code of the speechtotext action needs to be updated.