Refactored file system

Changed the file system to a more ordered state and updated links in
files accordingly. Also the reffering to files is now uniform

php/login.php

@@ -0,0 +1,32 @@
+<?php
+session_start();
+
+$username = $_POST['username'] ?? '';
+$password = $_POST['password'] ?? '';
+
+$usersFile = __DIR__ . "../data/users.json";
+if (!file_exists($usersFile)) {
+    die("Benutzerdaten fehlen!");
+}
+
+$users = json_decode(file_get_contents($usersFile), true);
+$loginOk = false;
+
+if (is_array($users)) {
+    foreach ($users as $user) {
+        if ($user['username'] === $username && $user['password'] === $password) {
+            $loginOk = true;
+            break;
+        }
+    }
+}
+
+if ($loginOk) {
+    $_SESSION['loggedIn'] = true;
+    $_SESSION['username'] = $username;
+    header("Location: ./admin.php");
+    exit;
+} else {
+    header("Location: ../pages/login.html?error=1");
+    exit;
+}