User signup and authentication

• Add DB models for user
• Add migration script
• Add sign-up UI
• Implement SSO (Google) on the front-end
• Write APIs to create/update user details
• JWT token to verify user auth (Stored as HttpOnly Cookie)

Closes: https://www.pivotaltracker.com/story/show/172690667, https://www.pivotaltracker.com/story/show/172690673