Make admin page redirect if you don't have permission.
Please note this code is untested. I do not have PHP development tools currently set up.
See the following for where I obtained the code to properly redirect: https://stackoverflow.com/questions/768431/how-to-make-a-redirect-in-php#768472

Reason for this PR:
Any user can access the admin.php page. This is unnecessary, and thus this commit fixes that. The usage of die() is to prevent malicious bots.