Add documentation for gunicorn and more testing

[daniviga]

We recently switched the Python server on production from uWSGI to Gunicorn due issues that we experienced with uWSGI and POST in the Vulnerability app (empty download cart). We'll need to update the documentation about uWSGI/Gunicorn.

We need also to do more testing using Gunicorn (by the way Icebox, which is the most critical piece, looks working well).