GeoPublisher / Zoom to layer extent (#495).

For schema plugins, a new template must be added in layout.xsl in order to extract extents for the metadata record (no extent may be returned if not applicable like ISO19110). eg.:

```
<xsl:template name="get-iso19110-extents-as-json">[]</xsl:template>
```
Merge remote-tracking branch 'upstream/stable-develop' into develop