Resolve "Improve/clean code"

Closes #6 (closed)

public/auth.php

 <?php
-session_start();
+require '../spotify/spotify.php';
 
-require '../vendor/autoload.php';
-require '../config.php';
-
-$session = new SpotifyWebAPI\Session(
-    $CLIENT_ID,
-    $CLIENT_SECRET,
-    $REDIRECT_URI
-);
+$session = new SpotifyWebAPI\Session($CLIENT_ID, $CLIENT_SECRET, $REDIRECT_URI);
 
 $state = $session->generateState();
 
 $_SESSION['state'] = $state;
 
 $options = [
-    'scope' => [
-        'playlist-read-private',
-        'playlist-modify-private',
-        'playlist-modify-public',
-        'user-read-private',
-    ],
+    'scope' => $SPOTIFY_REQUIRED_SCOPES,
     'state' => $state,
 ];