Server side geo lookup

Currently the admin javascript UI performs a geocode lookup (address to lat/long) on save. This creates a dependency on the BMLT maps API key to the admin javascript ui, and also seems to implicitly require the key restrictions to allow via referer (as the client IP addresses would be unknown).

The preference would be to provide this capability via a server side API, to avoid client side needing to know the maps key.

Am happy to tackle this as priority during the admin api rewrite if in agreement.