New subscriber endpoint (part 2)

[TylerMatteo]

Add the following functionality to the endpoint built in #1544

• [ ] Add a subscriptions object to the request body that can have properties for each of the 60 subscription topics.
  • [ ] The key for these properties should be the community district IDs found here - (K01, K02, etc). For citywide, use CW.
  • [ ] The value will be 1 to indicate a subscription
• [ ] Update the endpoint code to set the corresponding custom fields for the requested subscriptions to 1 on contact creation
• [ ] Set the "confirmed" custom field to 0 for the corresponding environment for the user on contact creation

Example of the expected request body for a user subscribing to districts K01, M01, and citywide:

{
  "email": "test@example.com",
  "subscriptions": {
    "K01": 1,
    "M01": 1,
    "CW": 1
  }
}

[TylerMatteo]

Dependent on #1560