M.me links with referral parameter don't work for first-time users

[rubycalling]

Describe the bug

When a first-time user click on a m.me link including a referral parameter and/or scan a QR code containing this type of link, they won't enter the expected curation flow, instead they will receive the NUX messages.

To Reproduce

Steps to reproduce the behavior:

1. Click on https://m.me/OriginalCoastClothing?ref=SUMMER_COUPON
2. Land on the Get Started page
3. Click the Get Started button
4. Observe that you receive the welcome message

Expected behavior

Instead of receiving the default welcome message that is usually triggered with the Get Started flow, you should get the summer coupon.

Additional context

The issue happen because we aren't reading the ref parameter value in the postback event:

{
  "sender":{
    "id":"<PSID>"
  },
  "recipient":{
    "id":"<PAGE_ID>"
  },
  "timestamp":1458692752478,
  "postback":{
    "payload":"<USER_DEFINED_PAYLOAD>",
    "referral": {
      "ref": "ref_data_in_m_dot_me_param",
      "source": "SHORTLINK",
      "type": "OPEN_THREAD",
    }
  }
}

See docs: https://developers.facebook.com/docs/messenger-platform/discovery/m-me-links#reading_parameter