Fix web UI index path http security headers

[Rendez]

The web-ui /dashboard endpoint can currently be embedded as an iframe. Adding the header X-Frame-Options: DENY shall protect against it.

In terms of the approach taken with actix_web, it would have been ideal if actix_files::Files allowed to set HTTP headers, but it only allows a handful of flags like etag, last_modified or content_disposition, which fall short of our needs.

This is why for the root path reading the index.html file a custom handler has been introduced: web_ui_index.

Awaiting for feedback in the approach before writing any kind of tests.

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