[Signup Wizard] Add 'userHasCompletedWizard' field to the Graphcool User schema

[dchao19]

This card is a child of #241.

This issue handles the backend logic necessary to route users to the signup wizard if they haven't completed it yet.

An accepted resolution for this issue would add some additional boolean (i.e userHasCompletedWizard) to the User schema in Graphcool and update full_schema.json accordingly.