Fixes #75 - move the release files on the file system

- [x] Upgrade script for relocating files to project ID directories
- [ ] Test the upgrade on copy of community data
- [x] Move also the plugin screenshots
- [x] Squash the "remove debugging code" commit
- [x] Remove ```/5000/<user_guid>/plugins/``` dir after upgrade if empty
Could you document why this handler is needed?