Revert "Live preview no longer experimental"

It is very much still experimental. The only thing it is capable of right now is rendering wikitext in the page and some basic skin enhancements (sidebar links and templates-used list).

By far it is not complete and lacks many things (such as interactive features like tablesorter and mw-collapsible. And things like preview-javascript on js user pages doesn't work etc.

This reverts commit 006916cd3f47bc07ce54830ab35520b69f2e87c1
Merge "Revert "Live preview no longer experimental""