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) 
ah, ok and so based on @rtibbles help, it looks like the original `by_zone` method will also return Devices that do not have a Zone. Is this the expected behavior or a bug? Guessing bug, but want to make sure. Either way, I need to put in some kind of fix so that we don't spit out device logs that aren't affliated with the zone the user exports for, and then this is ready for code review.