Feature Request: Option to disable preview (iframe) to original URL

## Type

- [ ] General question or discussion
- [ ] Propose a brand new feature
- [x] Request modification of existing behavior or design

## What is the problem that your feature request solves

Use iframe to preview original URL may leak `Referer` URL, cause JavaScript to be executed on browser and cookie leaved.
This may cause some problem related to security and privacy.

## Describe the ideal specific solution you'd want, and whether it fits into any broader scope of changes

Have an option to disable iframe preview to orignal URL, and/or option to remove `allow-scripts` from [`iframe.sandbox` attribute](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe#attr-sandbox)

## What hacks or alternative solutions have you tried to solve the problem?

Remove `iframe` tag completely from HTML template.

Also, I've sent a PR (#799) to resolve the `Referer` problem

## How badly do you want this new feature?

- [ ] It's an urgent deal-breaker, I can't live without it
- [x] It's important to add it in the near-mid term future
- [ ] It would be nice to have eventually

---

- [x] I'm willing to contribute [dev time](https://github.com/ArchiveBox/ArchiveBox#archivebox-development) / [money](https://github.com/sponsors/pirate) to fix this issue
- [ ] I like ArchiveBox so far / would recommend it to a friend
- [ ] I've had a lot of difficulty getting ArchiveBox set up

Start by locating the HTML template that emits the original-URL iframe and review PR #799 for the related Referer handling. Define how the preview-disable option and iframe sandbox setting should be exposed, then verify that the resulting HTML honors those settings without the unwanted preview behavior.

Written by the indexing model from the issue text.