PMC-882 Make the necessary changes in the core according to the "Laravel" upgrade guide (From v5.6.30 to v5.7)

composer.json

@@ -28,7 +28,7 @@
     "prefer-stable": true,
     "require": {
         "php": ">=7.1",
-        "laravel/framework": "5.6.*",
+        "laravel/framework": "5.7.*",
         "luracast/restler": "^3.0",
         "bshaffer/oauth2-server-php": "v1.0",
         "colosa/pmUI": "release/3.3.1-dev",

config/app.php

@@ -20,6 +20,7 @@ return [
         Illuminate\Queue\QueueServiceProvider::class,
         Illuminate\Translation\TranslationServiceProvider::class,
         Laravel\Tinker\TinkerServiceProvider::class,
+        Illuminate\Notifications\NotificationServiceProvider::class,
     ],
 
     'aliases' => [