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
Added example about configuring theme runtime
Fixes #6514: advanced application SignupForm::signup() should not return User model if saving failed
Security best practices guide
fix  mistake small  [skip ci]
Merge pull request #6519 from pana1990/patch-1

fix  mistake small  [skip ci]
A plan on security guide CSRF section
Fixes #6518 [skip ci]
[Uk] Guide corrections [skip ci]
[Uk] Guide corrections [skip ci]
Update start-looking-ahead.md

Adding the tag titled "yii2" on Stackoverflow to Community list.
Merge pull request #6522 from yiiframework-ua/uk_translate

[Uk] Guide corrections [skip ci]
Merge pull request #6523 from saidbakr/patch-1

[skip ci] Update start-looking-ahead.md
docs/guide/input-validation.md - typo [ci skip]
docs/guide-ja/input-validation.md - prepared for translation [ci skip]