New truncate and truncateWords methods

New truncate and truncateWords methods, https://github.com/yiisoft/yii2/issues/2941
Update BaseStringHelper.php

Length check.
Update CHANGELOG.md
Update BaseStringHelper.php

Updated to use mb_* functions
Test case for new truncate methods.

Test case for new truncate methods.
Removed app.php from vi messages, sorted languages
Added nl to languages
refactored filterWhere().
added filterWhere guide.
Fixed test break.
improved search model generator.
simplified search model [skip ci]
fixed search model to use base model class.
improved search model [skip ci]
Fixes #2955: Changed the signature of ActiveQuery constructors and `ActiveRecord::createQuery()` to simplify customizing ActiveQuery classes
fixed build break.
Fixed test break.
cleanup docs
cleanup CHANGELOGs
created build ReleaseController