Add json fixture for devices
Modify existing json fixtures slightly.
Remove xml fixture that is not being used.
Add manager methods to get objects by user.

The following model's managers have a getEditable method that takes a
PortcullisUser and returns the objects that user is allowed to edit:

Key, ScalingFunction, Device, DataStream, and PortcullisUser.
Add unit tests for models getEditable methods.
Change read_source to use objects.getEditable method.