Implement markdown, photos and videos

[TheRedScreen64]

Done in:

• [ ] Frontend
• [ ] Backend

Frontend note: https://www.npmjs.com/package/marked https://github.com/cure53/DOMPurify

Example:

document.getElementById('content').innerHTML = DOMPurify.sanitize(marked.parse('# Marked in the browser\n\nRendered by marked.'));