Switch to Git submodules for JS libs

This is motivated by : 

* A large number of plugins still under heavy development
* Easier tracking of upstream versions being used
* Prevent to modify external libs directly in source tree (yes, saw it with my own eyes)

It comes with a set of drawbacks : 

* Submodules have to be fetched. But ``clone --recursive`` is fine.
* Submodules repositories can disappear, be renamed...
* A lot of static files with be collected and not used. But we use django-pipeline or django-compressor...