Add sandboxed mode to HTML component

[wwwillchen]

Approach: This iframes HTML content (potentially from end-user input) in a sandboxed iframe without "allow-same-origin" to ensure the content does not run on the same origin as the main Mesop app.

This uses a double-iframing approach because we cannot directly use <iframe srcdoc="${html}"> from the main Mesop app because Mesop by default applies a stringent CSP (e.g. trusted types) and iframe documents using srcdoc automatically inherit their parent's CSP and this makes the iframe basically unusable for many use cases.

Instead we iframe sandboxed_iframe.html which is served with a minimal CSP that only prevents being iframed by other origins besides the Mesop app and then within sandboxed_iframe.html we render the (untrusted) HTML content inside a sandboxed iframe.

Fixes #156.

[richard-to]

LGTM - I like the api here. I think better than the alternative of having the embed element accept html.

[wwwillchen]

Need to hold off, there's a security issue with this approach.

[wwwillchen]

Need to hold off, there's a security issue with this approach.

Fixed now.