Feature: Ability to Login with Email

Thanks to @gsumpster for https://github.com/getgrav/grav-plugin-admin/pull/685

* implemented email-login, close #674
* changed placeholder text
* Fix Utils namespace thing
* Drop use
* Cleanup styling

classes/utils.php

@@ -0,0 +1,36 @@
+<?php
+namespace Grav\Plugin\Admin;
+
+use Grav\Common\Grav;
+use Grav\Common\User\User;
+
+/**
+ * Admin utils class
+ *
+ * @license MIT
+ */
+class Utils
+{
+    /**
+     * Matches an email to a user
+     *
+     * @return User
+     */
+    public static function findUserbyEmail($email)
+    {
+        $account_dir = Grav::instance()['locator']->findResource('account://');
+        $files = array_diff(scandir($account_dir), ['.', '..']);
+
+        foreach ($files as $file) {
+            if (strpos($file, '.yaml') !== false) {
+                $user = User::load(trim(substr($file, 0, -5)));
+                if ($user['email'] == $email) {
+                    return $user;
+                }
+            }
+        }
+
+        // If a User with the provided email cannot be found, then load user with that email as the username
+        return User::load($email);
+    }
+}