Use Content-Security-Policy header in Jahia's edit mode

[bpapez]

The goal of the spike is to
Investigate whether a content-security-policy could (soon) be used in Jahia's edit mode, or what still needs to be done.

Outcomes

• POC
• Story for next steps
• Documentation

Notes

For CKEditor5 integration (our richtext-ckeditor5 module is currently in beta), there is a CSP recommendation here: https://ckeditor.com/docs/ckeditor5/latest/getting-started/setup/csp.html