[ENGINEERING] Create User model and make new user route

[yxli001]

What is the issue?

• Create user model according to the docs
• Create a POST /api/users route - should also be protected by the auth middleware that Aaryan is going to make - that creates a new user document
  • UID is going to be in req.userId (put there by the auth middleware)
  • all the other fields should be passed in through req.body by whoever is sending the request
  • make sure to create a validator to make sure all the required fields exist and are passed in correctly (express-validator)
  • create a new User document with the sent in fields and return it with res.json()

Additional context