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

- cache all `/static/` files (except custom) for the lifetime of the server under normal circumstances
- resets cache on server restart
- don't cache any static file requests running from master (rely on 304 for caching content)
- don't cache files in /static/custom
- don't cache files in /nbextensions

This is much simpler than either #6580 or #6577. I do think we should consider compiling js, as in #6580, but I am skeptical of our ability to get it right in time for 3.0. This, however, seems pretty straightforward.