feature #1121 Updated to Symfony 5.1 RC2 (javiereguiluz)

This PR was squashed before being merged into the master branch.

Discussion
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Updated to Symfony 5.1 RC2

Everything works fine ... except the login form. When I enter the right credentials at `https://127.0.0.1:8000/en/login`, I'm redirected back to the login form without being authenticated.

@wouterj could you please check if there's something wrong on our side? These are the full logs of the failing login process. Thanks!

```
[2020-05-29T14:02:15.398605+00:00] request.INFO: Matched route "security_login". {"route":"security_login","route_parameters":{"_route":"security_login","_controller":"App\\Controller\\SecurityController::login","_locale":"en"},"request_uri":"https://127.0.0.1:8002/en/login","method":"POST"} []
[2020-05-29T14:02:15.420755+00:00] doctrine.DEBUG: SELECT t0.id AS id_1, t0.full_name AS full_name_2, t0.username AS username_3, t0.email AS email_4, t0.password AS password_5, t0.roles AS roles_6 FROM symfony_demo_user t0 WHERE t0.username = ? LIMIT 1 ["jane_admin"] []
[2020-05-29T14:02:16.452950+00:00] security.INFO: User has been authenticated successfully. {"username":"jane_admin"} []
[2020-05-29T14:02:16.458595+00:00] security.DEBUG: Stored the security token in the session. {"key":"_security_main"} []
[2020-05-29T14:02:16.544239+00:00] request.INFO: Matched route "admin_index". {"route":"admin_index","route_parameters":{"_route":"admin_index","_controller":"App\\Controller\\Admin\\BlogController::index","_locale":"en"},"request_uri":"https://127.0.0.1:8002/en/admin/post/","method":"GET"} []
[2020-05-29T14:02:16.546181+00:00] security.DEBUG: Read existing security token from the session. {"key":"_security_main","token_class":"Symfony\\Component\\Security\\Core\\Authentication\\Token\\UsernamePasswordToken"} []
[2020-05-29T14:02:16.551077+00:00] doctrine.DEBUG: SELECT t0.id AS id_1, t0.full_name AS full_name_2, t0.username AS username_3, t0.email AS email_4, t0.password AS password_5, t0.roles AS roles_6 FROM symfony_demo_user t0 WHERE t0.id = ? [1] []
[2020-05-29T14:02:16.551954+00:00] security.DEBUG: Cannot refresh token because user has changed. {"username":"jane_admin","provider":"Symfony\\Bridge\\Doctrine\\Security\\User\\EntityUserProvider"} []
[2020-05-29T14:02:16.552040+00:00] security.DEBUG: Token was deauthenticated after trying to refresh it. [] []
[2020-05-29T14:02:16.554066+00:00] security.INFO: Populated the TokenStorage with an anonymous Token. [] []
[2020-05-29T14:02:16.558520+00:00] security.DEBUG: Access denied, the user is not fully authenticated; redirecting to authentication entry point. {"exception":"[object] (Symfony\\Component\\Security\\Core\\Exception\\AccessDeniedException(code: 403): Access Denied. at symfony-demo/vendor/symfony/security-http/Firewall/AccessListener.php:117)"} []
[2020-05-29T14:02:16.559122+00:00] security.DEBUG: Calling Authentication entry point. [] []
[2020-05-29T14:02:16.666216+00:00] request.INFO: Matched route "security_login". {"route":"security_login","route_parameters":{"_route":"security_login","_controller":"App\\Controller\\SecurityController::login","_locale":"en"},"request_uri":"https://127.0.0.1:8002/en/login","method":"GET"} []
[2020-05-29T14:02:16.671891+00:00] security.INFO: Populated the TokenStorage with an anonymous Token. [] []
[2020-05-29T14:02:16.923429+00:00] request.INFO: Matched route "_wdt". {"route":"_wdt","route_parameters":{"_route":"_wdt","_controller":"web_profiler.controller.profiler::toolbarAction","token":"d40e08"},"request_uri":"https://127.0.0.1:8002/_wdt/d40e08","method":"GET"} []
```

Commits
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c6d1e7d Updated to Symfony 5.1 RC2

Author: javiereguiluz