Merge pull request
#4277 from minrk/jsext
serves files in IPYTHONDIR/nbextensions at /nbextensions.
This is a location for users / devs to drop-in frontend customization as js modules, css, etc.
The model is very much like the extensions dir (hence the name) - it's a location made available, but not loaded by default. You can load things from there with require, etc.
It's a configurable search path, just like static. This way we can define a system-wide location at a later point