Implement a user limit

Idea: being able to set a configuration to limit the number of users on an XWiki Instance.
Use-case: offer an hosting solution with some limits the administrator cannot change.

In practice:

• modify the "Users" section, in the administration, to handle this limit (already implemented on a side project)
• write a document save listener that cancel any save operation on a document holding a user if the limit is reached (already implemented on a side project).
• put the limit on a file on the filesystem so the "Admin" user cannot change it (or any other solution that achieve the same goal)
• put the configuration management in a component so that any hosting provider can override it with its own implementation