Issue #1843084 by effulgentsia, Crell: Convert user/register to Route.

.../Drupal/Core/EventSubscriber/FinishResponseSubscriber.php

core/lib/Drupal/Core/HtmlPageController.php

core/lib/Drupal/Core/Routing/PathMatcher.php

...ystem/lib/Drupal/system/Tests/Routing/PathMatcherTest.php

core/modules/user/lib/Drupal/user/UserRouteController.php

core/modules/user/user.module

core/modules/user/user.routing.yml