Changed all /px.gif to point to new beacon domain

Need to wait for @daithiocrualaoich opinion on the remaining references to /px.gif in the metrics gathering:

https://github.com/guardian/frontend/blob/master/diagnostics/app/conf/context.scala#L13
https://github.com/guardian/frontend/blob/master/diagnostics/app/metrics/NginxLog.scala#L2