Fixes in mkt.collections regarding automatic order

The automatic order wasn't working, it was kept at 0 everytime, for 2 reasons:
1. The `app=app` in the `CollectionMembership.objects.filter()` call made the queryset always return `order__max: None`. Removing the `app=app` fixes this.
2. The `aggregate + 1 if aggregate else 0` was always returning 0 if only automatic order was used. Starting the automatic order at 1 fixes this.

I also simplified the tests and added one with automatic order only.