The Customer model refactoring [FEATURE]

[bohdan-vorona]

Is your feature request related to a problem? Please describe.

We have two classes - common\models\Customer and frontend\models\Customer. The Order class uses the common\models\Customer class:

    public function getCustomer()
    {
        return $this->hasOne('common\models\Customer', ['id' => 'customer_id']);
    }

but in this task #180 I needed frontend\models\Customer since frontend\models\Customer has some additional methods like isLinkedToUser, getUserCustomer, and getUsers.

Describe the solution you'd like

Figure out why there are two different classes and potentially move the logic from frontend\models\Customer to common\models\Customer and leave only one class. Replace all the found places in the codebase with common\models\Customer instead of frontend\models\Customer.