In order to create the webapp creation route, I added to perform several
changes in the model, view, form and template.
I updated the `sandbox_state` with a default value to 2 (starting). It
correponds to the state when a webapp is created and the container needs
to be created by the controller.
I added specific choices corresponding to the possible states the
sandbox can be in. it avoids to enter any values that wouldn't be
interpreted by the controler.

I added a route for the webapp creation that can be accessed by
registered users (any registered users at the moment).

I created a new view relation to webapp creation. This view corresponds
to the creation of a new webapp object according to a form and the
creation of a quota object afterwards with default settings.
I added a link to redirect to the webapp creation form.
I added a template to display the form for the webapp creation route.
I added a form for the webapp creation.

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