Allow project's root in subdirectory

To allow, that the the project's root is not directly under the hostname but under a subdirectory, it is necessary that the icon references in sortable_tree.css.scss are used as
```
image-url("iconza/icons/red_add.png")
```
instead of
```
url(/assets/iconza/icons/red_add.png)
```

E.g. if the project's root is http://something.com/projectdir, image-url("iconza/icons/red_add.png") becomes url(/projectdir/iconza/icons/red_add.png).

See http://guides.rubyonrails.org/asset_pipeline.html chapter 2.2.2 CSS and Sass