Revamped workings of WYSIWYG code blocks

Revamped workings of WYSIWYG code blocks

Code blocks in tinymce could sometimes end up exploded into the sub
elements of the codemirror display.
This changes the strategy to render codemirror within the shadow dom of
a custom element while preserving the normal pre/code DOM structure.

Still a little instability when moving/adding code blocks within details
blocks but much harder to break things now.