Implementing POST user

To create a user, we are going to use the new_user and create_user functions. The new_user() function is relatively easy to implement; we just need to serve an HTML page with a form for a user to fill in.

Let's look at the steps:

1. Implement the new_user() function in src/routes/user.rs:

   #[get("/users/new", format = "text/html")]
   pub async fn new_user() -> HtmlResponse {
       let mut html_string = String::from(USER_HTML_
       PREFIX);
       html_string.push_str(
           r#"<form accept-charset="UTF-8" action="/
           users" autocomplete="off" method="POST">
       <div>
           <label for="username">Username:</label>
         &...