Send number of users in Active Installs ping data

The idea is to try to have some estimation of the number of users using XWiki in the world (as opposed to the number of xwiki instances. The number of instances is a relatively low number but can hide huge disparities. For example a single xwiki instance, counting only for 1 in our active instances can be used by 100K users and thus have a much bigger weight than another instance of XWiki being used by 1 person).

Total users = global users + all local users from all (sub)wikis.