I updated the model for the webapp creation. Now the sandbox version is
activated (though not used yet in the application). I updaed as well
the sandbox possible states
I did some naming modifications to be more coherent throughout the model
(removal of the `dj_` prefixes on foreign keys).
I adapted as well the script migration to take into account these
changes and save in the tree the django migration file.

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