[5.3] Move login attempt to separate method

[ivandokov]

This will make it easier to alter login attempt logic without the need to override the entire login() method. For example to change credentials or add other params. A good use case is if you are checking if someone is "admin".

<?php
namespace App\Http\Controllers\Auth;
//...
class AuthController extends Controller {
    // ...
    protected function attemptLogin(Request $request)
    {
        $credentials = $this->credentials($request);
        $credentials['admin'] = 1;

        return $this->guard()->attempt($credentials, $request->has('remember'));
    }
}

If this PR is accepted it will be great to make patch for versions 5.* because it is compatible.

[juukie]

Thanks! 👍