Add support for notebook server extensions

As some people may be tiring of my pointing out, it seems strange that
we have extension points for the JS and the kernel, but none for the
notebook server.

For cite2c, I want to add a handler which can serve a directory of style
files for use by the nbextension part.
Rename extensions -> server_extensions
Note that extension API is experimental
Merge pull request #7083 from takluyver/server-extensions

Add support for notebook server extensions