feat: Added lightbox/modal support for image preview

[TarikTz]

Add a simple "modal" class to images from the assets dialog and enable a modal window for preview. Clicking on the modal window will close it.

Example for a thumbnail preview with a clickable image

[![title](/example.jpg =300x)](/example.jpg){.modal}

or regular inserted image

![example.jpg](/example.jpg){.align-center}{.modal}