I changed the url field into a `URLField` instead of a `TextField`.
I defined as well a specific method that is used for debugging and in is
used in the admin. I commented it because it creates a bug into the
admin panel since the changes introduced in the model for the webapp
creation (the adding of the foreign key in the Webapp model).

Signed-off-by: Matthieu Berjon <matthieu.berjon@inria.fr>