Refactor install wizard and mail configuration

config/services.php

@@ -20,6 +20,11 @@ return [
         'domain' => env('MAILGUN_DOMAIN'),
         'secret' => env('MAILGUN_SECRET'),
         'endpoint' => env('MAILGUN_ENDPOINT', 'api.mailgun.net'),
+        'scheme' => env('MAILGUN_SCHEME', 'https'),
+    ],
+
+    'postmark' => [
+        'token' => env('POSTMARK_TOKEN'),
     ],
 
     'sparkpost' => [