The webapp creation is done in two steps. A first step that writes the
information in the database. Once the controller has created the
sandbox, the user is allowed to push its application on it and create a
new version.

I updated the models in order to satisfy the current state of the rails
database. I transformed the `sandbox_version_id` as a foreign key as
it's supposed to be in the rails `Webapp` model.
I updated the `WebappVersion` model to add the `state` possible choices.

I updated the webapp sandbox form in order to take into account the
`WebappVersion` model and add the related necessary fields.

I updated the `UserWebappListView` view to match the updated model. I
used as well a context object name to get a handle in the template with
a meaningful name.

The `WebappCreateView` now create an object in the database with all the
necessary data.

The `WebappSandboxCreationView` view create the sandbox and update the
Webapp model with the new version that has been committed.

I updated the templates in order to perform several specific actions.
I added a block called `meta` in the base template. The underlying idea
is to be able to supercharge on page basis the meta tags in the head of
the page. It used in the `webapp_creation_form.html` file where the
refresh every 5 seconds the page to read the database in order to know
when the sandbox is in its "running" state. Afterwards the meta tag is
removed to access the sandbox creation form in order to commit a new
version.

Resolves: #170



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