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
Merge pull request #7032 from minrk/require-url-arg

add '?v=<date>' to require URLs