Auto-Create groups: Ignore previously grouped users

MDL-45991 groups: Auto-create groups with ungrouped users only

When using auto-create groups, sometimes it is needed to only create
groups with users that are not already inside a course group. This
patch creates a checkbox that will ignore users that are in already
existing groups. It alters the groups_get_potential_members() function
so that the eligible users are retrieved entirely though sql.