BAC-550: move "served by" info to header

* remove ``<!-- Served by ... in ... sec -->`` comment (introduce ``$wgDisableReportTime`` global variable)
* add ``X-Backend-Response-Time`` response header with response generation time in ms
* add ``X-Served-By`` header with hostname of apache machine that generated the response (will be appended with Varnish machines handling client response)

@michalroszka / @prein / @Wikia/platform-team