- implement the deletion and restoration of webapp versions
- add the WebappVersionUpdate form to:
  - update version fields: number, description, published
  - restore a recently deleted version

- make WebappVersionList a pure list (remove the form) and add control
  buttons to edit/delete/restore the versions
- add the WebappVersion.deleted_at to record the deletion time
- rename the state 'REPLACED' as 'DELETED' (because versions can now be
  deleted explicitley by the user)
- use a common WebappVersionForm for the WebappVesionUpdate and
  WebappSandboxPanel views
- reuse the previous 'published' status by default when replacing
  a version in the sandbox panel
- add the 'ALLGO_EXPUNGE_DELAY' configure when the deleted images are
  destroyed for real