GHSA-5359-pvf2-pw78

### Impact
A [cross-site scripting (XSS)](https://owasp.org/www-community/attacks/xss/) vulnerability was discovered in TinyMCE’s content loading and content inserting code. A SVG image could be loaded though an `object` or `embed` element and that image could potentially contain a XSS payload.

### Fix
TinyMCE 6.8.1 introduced a new `convert_unsafe_embeds` option to automatically convert `object` and `embed` elements respective of their `type` attribute. From TinyMCE 7.0.0 onwards, the `convert_unsafe_embeds` option is enabled by default.

### Workarounds
If you are using TinyMCE 6.8.1 or higher, set `convert_unsafe_embeds` to true. For any earlier versions, a custom NodeFilter is recommended to remove or modify any `object` or `embed` elements. This can be added using the `editor.parser.addNodeFilter` and `editor.serializer.addNodeFilter` APIs.

### Acknowledgements
Tiny Technologies would like to thank Toni Huttunen of [Fraktal Oy](https://www.fraktal.fi/) for discovering this vulnerability.

### References
- [TinyMCE 6.8.1](https://www.tiny.cloud/docs/tinymce/6/6.8.1-release-notes/#new-convert_unsafe_embeds-option-that-controls-whether-object-and-embed-elements-will-be-converted-to-more-restrictive-alternatives-namely-img-for-image-mime-types-video-for-video-mime-types-audio-audio-mime-types-or-iframe-for-other-or-unspecified-mime-types)
- [TinyMCE 7.0.0](https://www.tiny.cloud/docs/tinymce/7/7.0-release-notes/#convert_unsafe_embeds-editor-option-is-now-defaulted-to-true)