Add device log resource option to CSV export

Adds a device log export option for central server admins. Columns:

name, description, id, last_sync, version, total_sync_sessions, deleted

@jamalex I added a `by_zones` class method to the Device model -- would love you to take a peek at that and make sure at a high level it makes sense before we commit it to secure sync, or if there's a way we can use the existing `by_zone` classmethod to do what I need to do (return a list of Devices that are associated with a list of one or more zone ids) 
Just chatted with @rtibbles. He might have been interpreting the issue differently.

To avoid duplication, perhaps the `by_zone` method could now be simplified down to:
`return self.by_zones([zone.id])`

And then this seems fine.