Generate Hubspot lead when new user account is created

[designmatty]

We need to implement some logic so we can start tracking leads in hubspot when users signup. The API call which we'll need to utilize is here: https://developers.hubspot.com/docs/methods/forms/submit_form

If you want a sense of complexity in terms of pointing out this issue see this issue: azavea/climate-change-api#888

To whoever ends up picking this up, please reach out to @designmatty for support.

[maurizi]

We currently don't track leads automatically on our internal Salesforce for Temperate - we post leads to an external Salesforce queue at our partner ICLEI-USA, which obviously we don't want to migrate to hubspot.

@jcahail is there any need to add hubspot tracking in addition to the tracking currently done through ICLEI?