Updated markdown preview to update on diff-basis

Uses vdom system to diff and update the current markdown preview view
instead of requiring a full HTML replace change.
This should provide better performance, expecially where dynamically
loaded content such as iframes were in use.

Closes #3454

package.json

@@ -28,6 +28,7 @@
     "dropzone": "^5.9.3",
     "markdown-it": "^13.0.1",
     "markdown-it-task-lists": "^2.1.1",
+    "snabbdom": "^3.5.0",
     "sortablejs": "^1.15.0"
   }
 }