Make `JsUrlThemeElement` and `CssUrlThemeElement` work with CSP in Jenkins 2.539+

### What feature do you want to see added?

https://www.jenkins.io/changelog/2.539/ added Content Security Policy as an opt-in feature. Previously users had to install [CSP plugin](https://plugins.jenkins.io/csp/) to get that.

New core extension points allow plugins to contribute to the CSP rule set.

I think it'd be great if this plugin allowed the configured JS element out of the box; although the exact shape of that is not that straightforward.

There's a limit to what the plugin can do automatically, and https://github.com/jenkinsci/simple-theme-plugin/pull/281 describes the limitations.