Write unit test(s) to ensure API Umbrella user is created when Apinf user signs up

[brylie]

When Apinf users sign up, we are

1. creating a corresponding user on API Umbrella
2. getting a returned user object
3. creating an ApiUmbrellaUser in Apinf
4. connecting the users via an ID field.

This process happens in accounts.coffee#52-72

Definition of done

• [ ] Write unit tests to cover each step in the above process.
• [ ] Run the tests to demonstrate success of the feature

[bajiat]

@mauriciovieira This could be an interesting case for integration test. Could you consider working on this after first unit test?

[mauriciovieira]

@bajiat yes. this would be a case for full end-to-end test using docker. Related (#2417)

[Nazarah]

If we have to replace API Umbrella with proxy 42 at some point, then the idea and associated works would be obsolete. Closing this.