added possibility to save messages to be translated

Best way to get the labels to search, the existing model.

Fixes #2016: removed ability to declare scopes in ActiveRecord leaving it to ActiveQuery

Conflicts:
	tests/unit/data/ar/elasticsearch/Customer.php

fixes #1585

fixes #1884

fixes #1905

* '1837-3rd-party-doc' of https://github.com/klimov-paul/yii2:
  Missing "md" extension added for "Using 3rd-Party Libraries" guide part
  "Using 3rd-Party Libraries" guide updated
  "Using 3rd-Party Libraries" guide added

Fix typo

fixes #2099

Simplify apidoc's RenderController::findRenderer to allow for custom renderers via ext

fixes #2094

PHPDocs: "app\models" changed to "common\models"

PHPDocs: "app\models" changed to "common\models"

Just a minor PHPDocs change. In the advanced application there is no such thing as "app\models".

Fix for #2087

active fixture for mongodb extension