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.
Add wait() to login test with wrong credentials to fix WebDriver test
Update BaseStringHelper.php
Added `HtmlResponseFormatter` and `JsonResponseFormatter`
typo fix [skip ci]
Update basics.md
Fixes
Update model.md
Merge pull request #3040 from MetalGuardian/update-model-md

Update model.md
Update module-debug.md
Merge pull request #3041 from MetalGuardian/update-module-debug-md

Update module-debug.md
Removed `Application::preload` in favor of `Application::bootstrap`
Merge pull request #3032 from arisk/patch-5

Update basics.md [skip ci]
updated phpdoc
renamed attributes to attributeNames in model

fixes #3034
Fixes #2034: Added `ContentNegotiator` to support response format and language negotiation