Add JSON/YAML import support for all objects

[jeremystretch]

Proposed Changes

Allow users to import objects in YAML or JSON format in addition to the existing support for CSV. YAML/JSON support was introduced for device types in #451; this extends that same functionality to all models.

Justification

JSON and YAML offer greater flexibility than CSV in incorporating related instances, as well as referencing parent objects.

[BarbarossaTM]

This is really cool, thanks!

Are there any plans to allow import of a full export with multiple items (for examples device types) at once? This would be really awesome!

Thanks and best
Max

[kjetilho]

One use case for this feature: The changelog holds the JSON object for deleted objects, and it would be very convenient if this could be fed directly into an Import Device dialog to get an undo feature. (Since the JSON contains numeric ids for stuff like site or device_type, it is not so convenient to recreate the device by hand.)

[jeremystretch]

Punting this to v3.2 as we'll likely need to refactor the current JSON/YAML import view substantially to accommodate models beyond device type.