Sign Up a User (Feature 0)

[bssammel]

Creates a new user, logs them in as the current user, and returns the current
user's information.

• New user exists in the database after request
• Successful response includes newly created id, firstName, lastName, email, and token
• Error response with status 403 is given when the specified email already exists
• Error response with status 400 is given when body validations for the email, firstName, or lastName are violated