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)

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)