Finish the sandbox panel

!94 (merged) has many TODOs/FIXMEs in:

django/allgo/templates/webapp_sandbox_panel.html
django/allgo/main/views.py (WebappSandboxPanel)

# TODO: check user permissions
# TODO: show error message if state is invalid

 <!-- FIXME: the current commit form is very crude (compared to
   the rails commit from we have in production)

   When we commit over an existing version, the previous version
   is completely overwritten (and not recoverable as of today).

   The form has to provide a clear feedback when the user is about
   to overwrite an existing version. In the rails app this is
   implemented as follows:
   - the commit form has two radio buttons: 'replace an existing
   version' and 'create a new version'.
     - when 'replace an existing version' is selected, the
     selection is done in a dropdown widget (that lists all
     existing versions)
     - when 'create a new version' the version input field is a
     regular text input field, but it prevents validating the
     form if it detects that the entered version already exists

   Also when we select an existing version, the 'changelog' should
   automatically filled with the existing text.

   for reference, this is implemented in:
   - rails/app/views/webapps/_sandbox_panel.html.erb
   - rails/app/controllers/webapps_controller.rb
 -->

{# FIXME: ask for confirmation before rollback (using a modal box) #}