[Security] Remove markdown-it-html5-embed

- Add tests for images with audio and video URLs.
- Remove `markdown-it-html5-embed`, which is quite old, depends on an outdated version of `markdown-it`, and loads in all of `mime-db` to check for audio or video URLs.
- Replace it with `lib/html5-embed`, a modified version of the original plugin, updated to use `mime/lite` to lookup MIME types for audio and video, in the browser.
- Bump `jsdom` and `isomorphic-dompurify`.