Fix CSRF bug in Jenkins form posts

Since we enabled CSRF protection for build Jenkins, we've been unable to successfully submit changes to many of the forms in the admin interface. For example, trying to suppress an irrelevant security warning via https://build.testeng.edx.org/configureSecurity/ results in the following error message:

The user is also logged out after attempting the form submission.

While we strive to make all changes to Jenkins configuration via code, it can be useful to make temporary changes via the admin interface. Diagnose why this isn't working and fix it.