support edition/deletion/restoration of webapp versions
(note: this PR is built on top of !205 (merged), you should accepte it first to ease the review)
This PR adds two new views: WebappVersionList and WebappVersionUpdate to allow the user to manage the version of his webapps.
- WebappVersionList lists the versions of a webapp and provide control buttons to edit/delete/restore the versions
- WebappVersionUpdate allows editing some version fields (number, description & published) and is also used for restoring deleted 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 & WebappSandboxPanel views
- use 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
- fix the main webapp index to only include the public webapps that have at least one version which is published and in state READY
close #266 (closed)