Support generating mercure URL and token directly in Twig

[alex-dev]

Using eventsource as a polyfill supporting headers for SSE, we can avoid issues with cookies.

• As cookies are shared between tabs, opening multiple tabs for different tasks will cause race conditions and break some mercure connections
• As cookies in the bundle don't allow configuring samesite, it is more code to support non strict modes
• As Symfony 5 doesn't even support cookie partitionning, using mercure in a iframe on a third party domain is impossible.

I suggest adding two methods to Twig extension: mercure_url and mercure_token. Both would not trigger cookie creation.