Add support for new "only global users" configuration in LDAP Authenticator

When a user from LDAP logs in the first time, the corresponginf XWikiUser object is crated on login.
This object happeneds to be created in the subwiki the user logs in to, even if that subwiki is supposed to have only global users.

Steps top reproduce:

• set up a fresh xwiki instance to use some ldap (in xwiki.cfg, so the settings are valid globally)
• create a subwiki with " Only global users are available in the wiki"
• after creation of the subwiki try to log in with a user from the LDAP

expected behaviour: the user gets logged in and is created in the global wiki
actual behaviour: the user is created in the subwiki only.

If the same user later tries to log in to the main wiki, a second user object will be created, with the same user id.

Admittedly the "global uses only" is mostly a UI restriction - everybody can create local users in such a subwiki manually. the main problem is that this happens automatically, and cannot be prevented.