Patch for #1528

This patch implements the suggestions outlined in #1528 - specifically, allowing the library used for layer preview to be toggled via a setting in `local_settings`.  This setting is used in the `layer_detail` template.

**Comment**

Although Leaflet is a popular and widely used library, GeoExt provides numerous useful tools for a more advanced user to explore a layer in more detail.  This patch also allows existing installations, whose users are perhaps more familiar with the GeoExt interface, to keep using it when they upgrade to the latest GeoNode.