User creation endpoint

[Robinnnnn]

[POST] /users

{
  username: string;
  bio: string;
}

• Should return a specific error code if the username already exists
• Prevent invalid characters for username: /^(?=[a-zA-Z0-9.]{2,20}$)(?!.*[.]{2})[^.].*[^.]$/;
• User bio is optional. It should be capped at 500 characters

We need to make sure to sanitize against XSS in bio, which should be handled by a library