R1:dba4c4bdf6a8

Emit a "Content-Security-Policy" HTTP header

Summary:
See PHI399. Ref T4340. This header provides an additional layer of protection against various attacks, including XSS attacks which embed inline `<script ...>` or `onhover="..."` content into the document.

**style-src**: The "unsafe-inline" directive affects both `style="..."` and `<style>`. We use a lot of `style="..."`, some very legitimately, so we can't realistically get away from this any time soon. We only use one `<style>` (for monospaced font…