Update webflux-form sample to use Thymeleaf Built in CSRF Support

[rwinch]

Summary

Thymeleaf provides automatic integration with Spring Security's CSRF support. We should update the webflux-form sample to demonstrate it.

• Update the dependencies to include both org.springframework.boot:spring-boot-starter-thymeleaf org.thymeleaf.extras:thymeleaf-extras-springsecurity5
• Remove the CsrfControllerAdvice
• Ensure the tests still pass by running ../../../gradlew check

[dbuos]

I'd like to take this one 😁

[rwinch]

Thanks @Daniel69 The issue is all yours! If you need any help, please don't hesitate to reach out to me

[dbuos]

Thank @rwinch, I'll start by reading https://github.com/spring-projects/spring-security/blob/master/CONTRIBUTING.md

[dbuos]

@rwinch I removed the CsrfControllerAdvice and added a dependency to org.thymeleaf.extras:thymeleaf-extras-springsecurity5:3.0.4.RELEASE because 'org.thymeleaf.extras:thymeleaf-extras-springsecurity' alone does not resolve, but two tests fail once I remove the Controller Advice (loginWhenInvalidUsernameThenError and loginAndLogout).
When I run the example alone after these changes and attempt to login, the only message that I can see is : 'CSRF Token has been associated to this client', no matter I use 'user' or 'invalid' as username.

[rwinch]

@Daniel69 Sorry for the delay getting back to you. The problem is that we aren't using Spring Boot 2.1.0.RELEASE yet. I created #6082 which will resolve this. Once it is updated to Boot 2.1.0.RELEASE you should be able to remove the version from org.thymeleaf.extras:thymeleaf-extras-springsecurity5 too

[rwinch]

Thanks again for your patience. Now that #6082 has been resolved, you should be able to rebase off of master and remove the version from thymeleaf-extras-springsecurity5.

If you need any help with this, please let me know.