Added user_checker support

[seblepers]

user_checkers in Symfony allow to verify if the identified user is allowed to log in : https://symfony.com/doc/current/security/user_checkers.html

This PR permits using user_checkers if configured.

[a-menshchikov]

@seblepers hi. Sorry for so late response!

Custom user checkers was introduced in Symfony 2.8, but 1.x branch provides support from Symfony 2.3. It's necessary to check Symfony version and use security.user_checker if it less than 2.8.