Refactor JS DOM context, get rid of "preloads" tree

- Refactor formstack iframed pages: remove JS, load CSS properly, and remove unnecessary preload wrapper /cc @rtyley 
- Get rid of "preloads", now calling it "js-context"
- ~~Remove grid bookmarklet, as no one seems to use it and it hasn't been working since the last JS refactor (where module names were changed)~~ (extracted to #3669)

Next step is to allow any child of js-context to break out of its parent horizontally.

![ ](http://media.giphy.com/media/SYut2vjrxt87C/giphy.gif)