Convert env() to config()

Laravel [recommends][1] only using `env()` within configuration files and using `config()` everywhere else. Doing so allows you to improve performance by running `php artisan config:cache`.

[1]: https://laravel.com/docs/5.7/configuration#configuration-caching

config/settings.php

@@ -0,0 +1,8 @@
+<?php
+
+return [
+    'mycelium_gateway_id' => env('MYCELIUM_GATEWAY_ID'),
+    'mycelium_gateway_secret' => env('MYCELIUM_GATEWAY_SECRET'),
+    'nntp_server' => env('NNTP_SERVER'),
+    'nocaptcha_enabled' => env('NOCAPTCHA_ENABLED'),
+];