Sign Up a User (Feature 0)

[Lillith13]

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

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