[JENKINS-69657] Un-inlining floatingBox.jelly for CSP compliance

[aneveux]

Hello 👋

Here's another Hacktoberfest Pull Request to solve JENKINS-69657, by un-inlining floatingBox.jelly, and then making a step forward towards CSP adoption in Jenkins.

Before submitting the PR, I tested the existing code, removed the script to show that my test was not working anymore, un-inlined the test and checked that I worked again. Here it is working with the script un-inlined (which is similar to the behavior before the PR):

Thanks a lot for your review, let me know if there's anything missing I can take care of 😄

Have a great day!

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