Add support for Symfony 3/4 in addition to Drupal

[fgm]

After fgm/accounts-drupal#20 the accounts-drupal package includes for raw Symfony backends in addition to Drupal 8.x.

However, this support currently needs proprietary code. The goal of this issue is to offer feature parity in an Open-Source Symfony package usable by Symfony 3.x/4.x as well as Drupal 8.x.