require PHP mcrypt extension

eg. used in /var/www/app/vendor/yiisoft/yii2/base/Security.php at line 465
Merge pull request #6497 from schmunk42/patch-1

require PHP mcrypt extension
Fixes #6505
use a monospace font in error details on exception page

better readability
Removed User.role from advanced application template because it was causing confusion that user could be assinged a single RBAC role and that role field is somehow tied to RBAC at all.
Fixes #6467: `ActiveForm` will scroll to the nearest visible element when the first error input is hidden