Add PLURAL support to youhavenewmessages*

Add PLURAL support on the number of talkpage messages received in
system messages youhavenewmessagesfromusers and youhavenewmessages.

Bug: 53897
Change-Id: Ic08c51f651fe4101ed0b6fcdaea4b4ae74331f70
Merge "Add PLURAL support to youhavenewmessages*"