Resolve "Webapp update template rewriting and logo hosting"

This merge request takes care of setting up the Cloudinary service in Django. It is used then to manage in the view the upload of a logo for a given webapp. This is achieved for the user through the webapp update form that has been extensively rewritten to be clearer and easier to use. I added as well a panel dedicated to the user role management (that will be added later on).

Closes #246 (closed), #169, #144 (closed)