Replace unmaintained Thickbox with other modal window in core

[maorb]

WordPress core uses Thickbox as a modal window.
Thickbox have officially announced this ​on their site:
While Thickbox had its day, it is not maintained any longer, so we recommend you use some alternatives. colorbox jQueryUI Dialog fancybox DOM window shadowbox.js

I think WP core should not use unsupported and unmaintained js plugin, and replace it with something else.

[SergeyBiryukov]

#10955