set no-cache header in StaticFileHandlers

rely on 304 Not Modified for caching content
add '?v=<date>' to require URLs

- resets cache on server restart
- don't cache requests on master (rely on 304 for content caching)
don't cache files in static/custom or nbextensions

always rely on 304 for cache content there
Load nbextensions from config
Extensions config for text editor
Merge pull request #7040 from takluyver/config-load-extensions

Load nbextensions from paths in frontend config
BUG: Prevent users from undoing the initial document load with CTRL-Z.

Prior to this change, setting a document's initial content in the text
editor created a history entry, which meant that hitting CTRL-Z
repeatedly could clear the entire document.  This change clears document
history after loading so that repeated CTRL-Z applications return the
document to its state at load time.
Merge pull request #7032 from minrk/require-url-arg

add '?v=<date>' to require URLs
Merge pull request #7046 from quantopian/fix-history-undo-load

BUG: Prevent users from undoing the initial document load with CTRL-Z.