Fixed confirmPOST JS utility not working with CSRF protection enabled.

Fixes (at least) the following issues:
* https://issues.jenkins-ci.org/browse/JENKINS-17977
* https://issues.jenkins-ci.org/browse/JENKINS-18032

I'm not entirely happy with adding the "crumb" unconditionally, which could be done only for POST requests. Is there any JS logic to check for safe methods (GET, HEAD, ...) that can be used here?

Any suggestions, feelings?