Uniform handling of javascript and CSS dependencies

We need to handle all of the external JS and CSS dependencies in a uniform way.

Currently, we have a variety of solutions used:

1) CDN based (e.g. jquery on google's CDN)
2) Copy and override (js and css are copied in our repository and overriden by additional code)
3) Submodule and override (we use git submodules instead of copying)
4) Copy and edit (we copy the needed files and modify them in place)

We need to find an uniform solution and a proper folder where to store external static dependencies (which plays well also with the static external dependencies provided by geonode).