Clean-up public dir

The public folder on latest master looks like this:

```
ckan/public
├── base
│   ├── css
│   ├── datapreview
│   ├── i18n
│   ├── images
│   ├── javascript
│   ├── less
│   ├── test
│   └── vendor
├── css
├── images
│   └── icons
├── img
└── scripts
    ├── dataexplorer
    └── vendor
```

What's public/scripts and public/base/datapreview? Can they go?
What's the difference between public/images, public/img and public/base/images? Same for public/css.