Have a means of marking HTML elements as safe to copy #1120
Description
When you have an HTML element in a question variable, it's easy to try to embed the same element twice in two different places.
It isn't always safe to copy an HTML element because it might have javascript event listeners attached to it.
It should be possible to mark an HTML element as 'non-interactive', which would mean it's safe to copy. Elements created with the html and table functions should have this mark automatically.
Then, when they're inserted into a content area, a copy would be taken.