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

- [x] Upgrade script for relocating files to project ID directories
- [x] Migrate also the plugin screenshots
- [x] Squash the "remove debugging code" commit
- [x] Remove ```/5000/<user_guid>/plugins/``` dir after upgrade if empty
- [x] Test the upgrade on copy of community data
- [x] Decide whether to move or delete project images when combining two projects
 - Currently they get removed when calling ```$old_project->delete();```
 - Currently the ```plugins/``` dir also gets left behind because it still contains the screenshots when calling ```rmdir()```
- [x] Find out if it's possible to easily change the release filename when combining
  - After combining plugin **foo** into plugin **bar** you would assume that the moved releases would be named **bar.zip** when you download them. The zip packages are however still named **foo.zip**
- [x] Update the commit message to better describe what the commit does