Make GeoIP script URL configurable

* Replace hard-coded '//bits.wikimedia.org/geoiplookup' with a
  configuration variable, $wgCentralGeoScriptURL, that is set to
  that value by default.
* When unset or false, no <script> tag will be injected.
* This fulfills a TODO :)
* With change I1a08c81da in place, we could set $wgCentralGeoScriptURL
  to false on the beta cluster.

Change-Id: I7154b8de671ed8cfcdf6d880eab9e34d09a21949
Merge "Make GeoIP script URL configurable"