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.
Update BaseStringHelper.php
Typecast model Id so that urls don't break when using MongoDB object Id's
Update view.md
Merge pull request #3141 from githubjeka/patch-3

Add static page
Adjusted statc pages docs
Added info about theming widget views. Fixes #3142
`yii\console\controllers\AssetController` fixed to respect data URL resources
`yii\console\controllers\AssetController::actionTemplate()` advanced to include `jsCompressor` and `cssCompressor` to the template.
added emphatises to phpdoc in asset bundle

issue #3143
Notes about `jsCompressor` and `cssCompressor` added to `assets.md`
Merge branch 'master' of github.com:yiisoft/yii2
Typecast model id only if using mongodb AR class
Fixed format
code style, fixes #3140
Code style and phpdoc fixes