Syncing models should not be in settings.py

"Syncing models" is logically owned by the syncing operation: securesync.  It is currently in settings.py (along with a helper function), which I think breaks the semantic organization of settings.py.  Syncing models is not an overridable option, and should not be touched by any user.

I made the following changes:
* Created a static SyncingModels class within the securesync/models.py file
* Moved storage of syncing models, and helper function, to this class
* Made static procedures from securesync/models.py a member of this class (e.g. save_serialized_models).
* Did something similar for get_device_counters and the Device model class.

I think avoiding having unassociated procedures is conceptually helpful, given the object-oriented Django framework.  I think each of these was a natural fit, so worth making the change for.  Should also help documentation be a bit cleaner, when I write it.