Expose JQUERY_UI_FILENAME for templates

The setting is useful to load jQuery UI from a template in a general manner, so lets make it useable in this way.

For example one can then add:
```HTML+Django
<link rel="stylesheet" href="{% static
    "mezzanine/css/smoothness/"|add:settings.JQUERY_UI_FILENAME|slice:":-2"|add:"css"
%}">
...
<script src="{% static "mezzanine/js/"|add:settings.JQUERY_UI_FILENAME %}"></script>
```
to a template to avoid a dependency on a concrete version of the library.