Feature: Sign-Up

[huanglallen]

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

Backend:

• New user exists in the database after request
• Successful response includes newly created id, firstName, lastName,
  and email
• Error response with status 500 is given when the specified email or username
  already exists
• Error response with status 400 is given when body validations for the
  email, firstName, or lastName are violated

Frontend:

• Sign-up button opens a modal
• Sign-up modal should contain the input fields: firstName, lastName, email, password, confirm password, and a submit button
• When pressing submit, errors will display when errors exist
• On a successful Sign-up, modal will close and User should automatically be logged in