Add app_url for mail sender (pterodactyl#3753)

Co-authored-by: Dane Everitt <dane@daneeveritt.com>

Author: NotYusta

.env.example

@@ -8,6 +8,7 @@ APP_DELETE_MINUTES=10
 APP_ENVIRONMENT_ONLY=true
 LOG_CHANNEL=daily
 APP_LOCALE=en
+APP_URL=http://panel.example.com
 
 DB_HOST=127.0.0.1
 DB_PORT=3306
@@ -30,7 +31,7 @@ MAILGUN_ENDPOINT=api.mailgun.net
 # mail servers such as Gmail to reject your mail.
 #
 # @see: https://github.com/pterodactyl/panel/pull/3110
-# SERVER_NAME=panel.yourdomain.com
+# SERVER_NAME=panel.example.com
 
 QUEUE_HIGH=high
 QUEUE_STANDARD=standard