SESSION_SECRET default setting improvement

This PR improve the error raised by the server when there is no SESSION_SECRET set.
Moreover there is now only one runserver target for the Makefile:
    
    make runserver # development
    NODE_ENV=production make runserver # production mode
    
Also these patches takes care of setting a default SESSION_SECRET for development mode. But if we launch the server in production mode without explicitly setting SESSION_SECRET, it will raise an error. The intent is to prevent setting a dumb secret without noticing it.