Added a debug toolbar panel for MongoDB
`yii\mongodb\log\MongoDbTarget` log target added
Fixes #4048: Added `init` event to `ActiveQuery` classes
Improved checks where substr_compare is used, replaced more substr + comapre cases with substr_compare
property code style extensions
Using of `MongoCollection::createIndex()` provided
Fixes #4526: fixed error caused by substr_compare length parameter doesn't accept null in PHP 5.4
improved IDE autocompletion for AR::find()

[ci skip]
Fixes #4708
`yii\mongodb\Collection::buildLikeCondition()` fixed to be case insensitive.
Fixes #4755: `yii\test\BaseActiveFixture::unload()` does not clean up the internal cached data
Fixes #4914
merge with upstream
Dynamic naming added to `DbPanel`
`MongoDbPanel` addition mentioned at CHANGELOG.md and README.md
Fixed `yii\mongodb\Collection::buildInCondition()` handles non-sequent key arrays
added branch alias to composer.json
composer.json tabs -> spaces
Fixes #5176: `ActiveFixture` will reset table in its `load()` method instead of `unload()`
prepare for 2.0.0-rc release.