* Added 'return' to goHome() calls in actions
* Changed $_POST to Yii::$app->request->post()

Added in an isset check because if you set a flash type that is not in our expected index it will throw a undefined index error

This reverts commit c94131b8.

Fixes #2341, fixes #2317: moved codeception dependecies to "suggest" section, fixed codeception version on stable branch