Warn the wiki administrators when xx% of the user / wiki quota is reached

[aubincleme]

As an admin using a pro app, I don't want to be surprised of my license not being applicable anymore because a new user joined my wiki, or because someone created a new subwiki.

A solution for this would be to add a warning either by email to the members of the XWikiAdminGroup or in the notification center (as it is the case today when a license is expired), indicating that there are only X wikis left to create or Y users left to register before running out of slots for the current license.