Allow specifying a group format and cleaning group names

We need to be able to specify a group format so that for instance a "Group" suffix can be added to confluence group names.

We also need a way to clean up group names.

See https://github.com/xwikisas/application-confluence-migrator-pro/issues/65

To this effect, we add a "Group Format" parameter in which ${group} and ${group._clean} placeholders can be specified, following the format used in the LDAP application, so Confluence group can be imported using XWiki conventions.