allow env vars and web.xml to trump cfg props
display alternate wicket ui on external api key
delegate to GeoServerExtensions.getProperty

Instead of manually checking environment variables, delegate to the
GeoServerExtensions.getProperty utility method to do so. This changes
the lookup order, but is now consistent with other geoserver lookups.
We also don't know the source of the override, just whether an override
exists or not. Update the interface and UI accordingly.