Marking a place as pending on a non-queued event show incorrect places left

[nukeador]

If an event has places left and you mark from the admin a registration as "Mark as pending and send an email for confirmation", the placesLeft counts goes zero due:

https://github.com/mozillahispano/mozevents/blob/master/events/models.py#L45

We should find a way to avoid a registration to be marked as pending if the event has no queue.

https://github.com/mozillahispano/mozevents/blob/master/events/admin.py#L98