[fix bug 991553] excluding invisible groups from UI

Before, we had the members_count as annotation into the queryset returned by both Group and Skill. So the idea of this change is to avoid to override get_query_set on GroupManager with the only reason of returning a custom QuerySet.
If we remove this line we would need the following:

```
class GroupBaseManager(Manager):
  def get_query_set(self):
    qs = super(GroupBaseManager, self).get_query_set()
    qs = qs.annotate(member_count=Count('members'))
    return qs

class GroupQuerySet(QuerySet):
  def visible(self):
    return self.filter(visible=True)

class GroupManager(GroupBaseManager):
  def get_query_set(self):
    qs = GroupQuerySet(self.model, using=self._db)
    qs = qs.annotate(member_count=Count('members'))
    return qs
```

which would duplicate the line:
```
qs = qs.annotate(member_count=Count('members'))
```

thoughts?