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

- cache all `/static/` files for the lifetime of the server under normal circumstances
- resets cache on server restart
- don't cache static file requests running from master (rely on 304 for caching content)

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, is quite straightforward.

As this PR currently stands, this will make caching nbextensions and/or custom.js a bit worse for users not living on master, because it tells the browser that these files will not change for the lifetime of the server, which may not be true. I can add special handling of URLs that start with `/custom` and `/nbextension` easily enough, since those are the only locations of files that might **reasonably** change during the lifetime of the server. We just have to decide that's a good idea.