Allow to disable LDAP group synchronization

The goal of this issue is to add a new option to the property ldap_mode_group_sync : never. When set, the user group membership should never be synchronized automatically when the user is authenticated.

The reason for this change is to allow wikis to work with LDAP group mappings performed through third-party tools. The synchronization could be either asynchronous (avoiding performance issues when working with very large LDAP directories) or be based on another way to manage group sin LDAP.